In a large bowl, mix together the baking soda, salt, citric acid, and cornstarch and stir until thoroughly combined.
Melt the coconut oil if necessary and stir in the fragrance or essential oils of your choice.
Slowly and carefully combine the wet ingredients with the dry ingredients. Be careful not to add too much liquid at once or the citric acid reaction will start to bubble and fizz! Squeeze the bath bomb mixture in your hand to test if it will stick together. If not, spray the mixture with 1-2 spritzes of water or witch hazel, mix well, and test again. Repeat until you reach the desired consistency. You want the mixture to hold together when compressed and compacted, but DO NOT GET THE MIXTURE TOO WET!
Fill half of the mold with the bath bomb mixture, pressing down to ensure that it is completely filled. Fill the other half of the mold, and press both filled halves tightly together.
Allow to dry for at least 24 hours before unmolding. I recommend drying for an additional 24 hours after unmolding before handling, decorating, and gift packaging (if time allows).
To make the snowman faces, mix a small amount of charcoal powder with rubbing alcohol at a 2:1 ratio. Use a paintbrush to carefully add black “coal” dots to the face. Repeat the process with the orange mica to create the snowman’s carrot nose.