In a small mixing bowl, whisk together the cornstarch and water until smooth and lump-free. Stir in the creamy peanut butter and honey. Mix well until fully combined. The mixture should have a smooth, spreadable consistency.
If you’d like to add color, stir in a few drops of natural, pet-safe food coloring. Mix until the color is evenly distributed.
Use a piping bag, spoon, or spatula to spread or drizzle the icing over cooled dog treats or cake. Let it set for 10–15 minutes. For a firmer set, place it in the refrigerator.