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    Decorated Treats » Designs & Tutorials

    Rainbow Birthday Cake

    Published: Sep 19, 2020 · Modified: Aug 5, 2022 by Priya Maha

    This rainbow cake makes a pretty birthday cake with a 2D fondant rainbow on the side and lots of fondant clouds at the base of the cake, the sides as well as on the top. And to add more rainbow colors to the birthday cake, the "HAPPY BIRTHDAY' letters are in rainbow colors too!

    A round blue cake decorated with rainbow and clouds on the sides and rainbow alphabets on top of the cake.
    Jump to:
    • 🧾Supplies Required
    • 👩‍🍳How to Make
    • ❤️More Cake Tutorials You Will Love

    🧾Supplies Required

    • Cake - This was a single tier cake, measuring 6 inches in diameter. The height of the cake was also 6 inches. Inside, the birthday cake consisted of 3 layers sandwiched with buttercream. I used my marble cake recipe for the cake. Since the recipe calculations on my recipe page were meant for cakes with a height of 4 inches, I multiplied the recipe by 1.5 times.
    • Buttercream - I used buttercream to fill in between the 3 cake layers as well as to crumb coat the cake. My buttercream recipe can be found here.
    • Fondant - I used Satin Ice fondant in white and baby blue. The baby blue fondant was to cover the birthday cake, while the white was for the clouds. I also used fondant for the rainbow letters on the cake as well as the 2D rainbow on the side of the cake.
    • Shortening and fondant glue - I used both these mediums as glue on the birthday cake. 
    • Satin ribbon - I used a light blue satin ribbon which I purchased from a local craft shop to wrap around the cake board. 

    👩‍🍳How to Make

    Step 1: Make the Rainbow Fondant Letters

    I made these letters in advance to allow them sufficient time to set and harden. I have a separate tutorial with detailed step by step instructions on how to make the rainbow fondant letters. Here is the summary:

    Pushing rainbow colored fondant strips together with hand.

    Mix the fondant into rainbow colors and use the extruder tool to pipe out long strips.

    Using a sugar craft knife to cut the fondant on which in a letter H cutter.

    Arrange and attach the strips with glue. Cut the letters with the colors running diagonally for each letter.

    Using a needle tool to remove excess fondant of a fondant alphabet cutter.

    Remove the letters from the cutters. Use a needle to remove excess fondant on the sides.

    Fondant letters in rainbow colors.

    Let the letters set completely in a horizontal position before using.

    Step 2: Make the 3D Fondant Clouds

    I also made these in advance as they are placed upright on the cake and need to set and harden before being placed on the cake. I have a detailed tutorials on how to make the 3D fondant clouds. Here is the summary:

    Cloud paper templates on a piece of rolled fondant, and using a sugar craft knife to cut them out.

    Roll some white fondant thick and use paper templates to cut the clouds out.

    Showing the bottom of a fondant cloud by hand.

    The clouds need to be thick so that they can stand upright on the cake. Let them set completely in a horizontal position before using.

    Step 3: Bake and Cover the Cake with Fondant

    Closer to the delivery day, I baked the cake, sandwiched the layers with buttercream, crumb coated, and covered it in Satin Ice Baby Blue fondant. I also covered the cake board in the same color.

    Step 4: Make the Fondant Rainbow

    Next, I attached the 2D rainbow on the side of the cake. I used the same colors as the rainbow letters and used my extruder tool to pipe out consistent-sized strips for the rainbow. Click here for a detailed tutorial on how to mix the rainbow colors and how to make the 2D fondant rainbow). Here is the summary:

    Fondant arranged in a row, in the 7 colors of a rainbow.

    Mix fondant into the colors of the rainbow. Use the same colors as the alphabets in step 1 above.

    Using a round cutter to position a violet fondant strip on the side of a cake.

    Pipe the fondant into strips with an extruder tool. Use a round cutter to guide the placement of the first color.

    Fondant rainbow on the side of a light blue fondant covered cake. The red strip of the rainbow is longer than the rest of the colors.

    Glue the remaining colors to the cake and trim off excess fondant.

    Fondant rainbow on the side of a light blue fondant covered cake.

    This is bow the completed rainbow should look like.

    And then I wrapped a blue satin ribbon all around the cake board.

    A cake placed on a cake board covered in blue fondant and wrapped with blue satin ribbon on the sides.

    Step 5: Attach the Rainbow Letters to the Clouds

    After the rainbow and the ribbon, I worked on the rainbow 'HAPPY BIRTHDAY' letters on the cake. The letters 'HAPPY' were positioned on the largest 3D cloud on the cake. The alphabets 'BIRTHDAY', on the other hand, were glued on the cake itself.

    Before gluing the rainbow letters, I arranged them on the cloud and made sure they were centered.

    A rainbow colored letter P on a white fondant cloud. On the sides is the letters H, A, P and Y.
    The word HAPPY in rainbow colors on a piece of cloud shaped white fondant.

    Similarly for the word 'BIRTHDAY', to make sure they were perfectly centered, I arranged them in a lying down position as shown below. And then I adjusted the position until the letters were well centered.

    Using a brush to brush the bottom of a rainbow colored letter H.
    Rainbow alphabets in fondant placed on a cake.

    Only then did I glue them one by one, starting from the middle, with fondant glue.

    A blue cake decorated with 2D rainbow on the sides and rainbow alphabets on top.

    Step 6: Add the 2D Fondant Clouds

    Next were the clouds. I have a separate detailed tutorial on how to make these fondant clouds. Here is the summary:

    Small balls of white fondant.

    Pinch white fondant and shape in balls in random sizes.

    3 patches of white fondant at the border of cake decorated with fondant rainbow.

    Flatten the balls slightly and attach them to the cake, below the rainbow.

    A cake decorated with fondant rainbow and a border of white fondant patches as the clouds.

    Overlap some of the balls to make clusters.

    A group of fondant clouds on a cake.

    Repeat the same to make cloud clusters above the rainbow.

    Step 7: Attach the 3D Cloud Toppers

    And finally, the 3D clouds went on. I started with the largest one and attached it to the cake with some fondant glue.

    Once dried, I accented them with clusters of fondant balls on the cake.

    A round blue cake decorated with rainbow and clouds on the sides and rainbow alphabets on top of the cake.

    And that was it. My rainbow birthday cake. 

    A round blue cake decorated with rainbow and clouds on the sides and rainbow alphabets on top of the cake.

    ❤️More Cake Tutorials You Will Love

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      Butterfly Pea Cake with Lemon Cream Cheese Frosting
    • A round rosette cake in white, pink, burgundy and green.
      Rosette Cake (Buttercream Cake Recipe)
    • A round black cake with Star Wars themed cake.
      Star Wars Birthday Cake (How to Make)
    • A round double barrel cake decorated with Simba cake topper, Lion King edible image and green fondant palm leaves.
      Lion King Cake with Simba Cake Topper

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    A round rainbow themed cake.