Preheat oven to 175°Celsius. Line baking trays with parchment paper.
Start by making the sanding sugar. Divide the coarse sugar equally into 2 bowls. Add a drop of the turquoise food color in one bowl and the green food color in another (see Note 1).
Mix the sugar with a spoon until the color is all well distributed. Add more color if you wish to have a deeper color. Leave the colored sugar aside to dry.
In a separate medium sized bowl, add the butter (see Note 2). Sift in the flour and mix both on low speed until it turns crumbly. You can also use a pastry cutter to cut in the butter into the flour or rub it in with the tips of your fingers.
Stir in the sugar and mix well, followed by the vanilla extract. The mixture would still be crumbly at this stage.
Turn the dough onto a clean workspace and press it together to form a ball (see Note 3).
Divide the dough into smaller sections and roll each section between 2 parchment sheets. Use 2 dowel rods on either side of the dough when rolling for consistent thickness (see Notes 4 & 5)
Peel off the top parchment sheet and cut out the cookie dough with a cookie cutter. Carefully lift the cut cookies, with a flat spatula, neaten the edges and transfer onto the baking tray (see Note 6).
Sprinkle the colored sanding sugar on top of each cookie and press it in softly with your fingertips.
Bake the butterfly cookies for approximately 12 to 15 minutes until the sides of the cookies start to turn light gold.
Remove cookies from oven. Let them rest in the tray for a few minutes before transfering them onto a wire rack to cool completely.
Store the decorated butterfly cookies in an airtight container.