In a small bowl, combine the Greek yogurt and peanut butter. Stir until smooth.
Sprinkle in the cornstarch (or tapioca starch) a little at a time. Mix after each addition until the icing thickens into a soft, spreadable texture.
Divide your icing into separate bowls if you want multiple colors. Add small amounts of natural coloring.
Spread the icing with a spoon or use a piping bag to create adorable designs. Let the treats chill in the fridge for about 30 minutes.
