In a large bowl, combine baking soda, citric acid, cornstarch, epsom salt and a small amount of cosmetic pigment powder (less than a tablespoon) or a few drops of gel food coloring.
Combine coconut oil and essential oil then stir into dry ingredients.
Spray a few pumps of witch hazel until the mixture can hold its shape.
Firmly press into silicone baking cup. Let bath bomb dry overnight before gently removing from baking cup. Tip: lay a soft towel down to gently drop the cupcakes on.
For the 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 needed.
Pipe on top of bath bombs and immediately add sprinkles. Let frosting dry overnight before packaging.