In a bowl combine the dry ingredients (baking soda, cornstarch, epsom salt and dried lavender).
Whisk to remove any clumps. You want the mixture to be very fine, so rub any lumps together with your fingers to remove them if necessary.
In a small bowl, mix together the water and coconut oil. If you're using liquid food coloring, add a couple of drops here. I used powdered, plant-based food coloring so I added it at the very end.
Slowly pour the liquid mixture in with the dry mixture, whisking as you go. If you see the mixture begin to fizz or foam, you're adding it too quickly.
Add about 20 - 30 drops of essential oil, depending on how scented you want your bath bombs.
Once all the wet ingredients are combined with the dry, take a small amount in your hand and squeeze it together. It should stick together, almost like damp sand. If the mixture is still a bit powdery, add another drop of water.
If you're using powdered food coloring, now is the time to add it. Sprinkle some and whisk it together with the rest of the ingredients.
Fill each mold with the mixture until they are slightly overfull. Then press the molds together.
Allow to bath bombs to dry for an hour before trying to remove the mold. You want them to dry outside of the molds because they will stick and crack otherwise.
If after an hour, they break when taking them out of the mold, just stick them back together and allow to dry for another short while. You can also pop them in the fridge for a few minutes.
Once the bath bombs have been removed from the molds, allow them to dry on a tray overnight.
Once dry, store in a Ziploc baggie or pop them in some cellophane for a pretty gift!