Mix baking soda, oatmeal & citric acid together. Set aside.
Melt cocoa butter in microwave in a glass container. Add dry ingredients to melted cocoa butter. Add essential oils.
Pour in ice cube trays or small molds. Place in freezer for 30 minutes.
Store in glass jars or plastic bags. Keep cool.
Note: What you are getting is melted cocoa butter that has solidified with the baking soda and citric acid binding it together. The molds you choose won't matter like they do for bath bombs. But -- don't make the molds too big or you are are going to waste product and your bath will be a sludge. I found ice cube trays that are shaped like little balls. You don't want any more than 1 - 1-1/2 tsp of product per cube. I have found that Ikea sometimes has really cute ice cube trays shaped in stars, hearts and circles with holes in the middle. Or.if you can get your hands on them -- candy molds would work very well, you would just need more for your bath.