Combine icing sugar and ½ cup cornstarch in a medium bowl and stir with a spoon.
Add frosting and stir until combined. If needed, continuing kneading with your hands until the dry ingredients are fully incorporated.
Add more cornstarch, one tablespoon at a time, and knead until play dough is soft enough to play with and no longer feels sticky.
Keep kneading until the dough feels super soft and smooth. Add a few drops of glycerin if desired to make it even smoother, then add chocolate sprinkles (optional) and start playing immediately.