Sift the cornstarch and powdered sugar and mix them together in a bowl. Set aside.
Prepare a 6 inches square tin. Grease the tin with oil, line with parchment paper and grease again. Dust thoroughly with some of the powdered sugar and cornstarch mix. Set aside.
In a separate bowl, mix the gelatin with half of the hot water and leave aside for the gelatin to bloom.
In another saucepan, mix sugar with the remaining half of the water (¼ cup) and cook over medium heat until the sugar is dissolved.
Turn the heat to low and continue to cook the syrup until it reaches the soft ball stage (see notes below) or until it reaches between 235 to 240°Fahrenheit or between 112 to 116°Celsius on a candy thermometer.
Pour the hot syrup into the gelatin mix and stir until gelatin is all dissolved. Let it cool down slightly for about 2 minutes.
Whisk the gelatin mixture on low speed for 3 minutes before increasing the speed to high for another 10 minutes. The mixture should be white and glossy but not too stiff. Add vanilla extract and continue to whisk for another minute. If you wish to color your marshmallows, add the coloring at this stage.
Pour the marshmallow mix into the prepared tin and level the top with a spatula.
Leave the marshmallow to set at room temperature for at least 8 hours or overnight.
Carefully lift the marshmallow block out of the tin. Peel off the parchment lining and dust the uncut marshmallow block with the remaining powdered sugar – cornstarch mix. Grease your knife and cut the marshmallow block into smaller pieces as you wish.
This homemade recipe makes a 6 inches square marshmallow block with a height of about 1.3 inches.