Combine 1 cup baking soda, 1/2 cup citric acid, 3 tablespoons Epsom salt, and 1/2 cornstarch in a large bowl using a whisk to make sure to combine.
In a glass or metal measuring cup, combine coconut oil, 5 drops of soap food coloring, 20 drops of grapefruit essential oil.
Stir the oil mixture into dry ingredients.
Add Witch Hazel and 5 drops of soap food coloring.
Spray the witch hazel mixture into the bowl with all the other ingredients. Keep mixing and spraying until the mixture can hold its shape. Don't let the mixture sit with the spray on it. You want to spray and mix quickly before it starts fizzing. *** Note: Do not pour in the witch hazel because it will cause the chemical to start prematurely.
One the mixture holds its shape, press it firmly into silicone baking cup.
Let cupcake bath bomb dry overnight before gently removing from silicone baking cups.
For the cupcake bath bomb icing:
Combine meringue powder, powdered sugar and warm water in a stand mixer. Beat for 7-10 minutes or until icing is stiff.
Add half a tablespoon of water at a time. If you are making the colored icing, you can mix the soap food coloring in instead of the water first.
Pipe on top of bath bombs and add sprinkles immediately so that they stick.
Let frosting dry overnight before packaging.