Mix together baking soda, cornstarch, citric acid and epsom salts.

Add the fractionated coconut oil, then quickly mix in the water (so it won't fizz too much).

Divide the bath bomb mixture into two bowls.  roughly ⅓ in one bowl and ⅔ in the other. Mix the vanilla essential oil into the smaller batch and add the lemon essential oil and some yellow food coloring to the larger portion.  Mix each well, taking care not to get any yellow into the white.

Scoop the mixtures into each side of the bath bomb mold in rough layers. Press firmly until nearly flush, then overfill with mixture and press the two sides together snuggly.  Gently twist and lift top to release (if the halves of the bomb separate that means you probably didn't pack tightly enough)