Birthday Cake Marshmallows

Name: Birthday Cake Marshmallows
Cuisine: American
Ingredients: 210 milliliters water, divided (about 1 cup, divided); 20 grams powdered gelatin* (2 tablespoons); 400 grams granulated sugar (2 cups); 200 grams corn syrup (1/2 cup); 1/4 teaspoon salt; 1/4 – 1/2 teaspoon cake batter flavoring; Yellow gel food coloring; 3 tablespoons rainbow nonpareil sprinkles; Canola oil or nonstick cooking spray, for greasing tin and knife; 50 grams potato starch* (1/3 cup); 50 grams confectioner’s sugar (1/3 cup); Rainbow nonpareil sprinkles, for coating top (optional)