In a large glass or metal mixing bowl, combine baking soda, citric acid, corn starch, and coconut oil. Mix well.
Add pumpkin pie spice, pumpkin flavoring, and soap colorant. Mix well until all of the ingredients are well incorporated. The consistency will look and feel like wet sand.
Spoon the mixture into the mold, packing it down firmly and filling each cup to the top.
Place the mold on a baking sheet and into the oven. Turn off the oven and allow the mold to sit in the warm oven for 1 hour.
Remove from oven and allow to air dry for 6 to 8 hours.
Gently remove each bath bomb from the mold and transfer to a glass container for storage.
Notes
*Coconut oil: Some coconut oil comes in liquid form, others in solid form. If your coconut oil is in solid form, melt it in the microwave for 20 to 30 seconds before adding to mixture.