Mix the baking soda, citric acid and milk powder together.
Add the vanilla oil and coconut oil. Mix well. Then add the water while stirring. Once well mixed test to see that a gasped handful will lightly clump, if not add another 1/2 tsp of water, mix and try again until it will.
Overfill each half of the bath bomb mold considerably, then press firmly together. Gently remove the mold and set the bath bomb aside to dry out.