Add baking soda, epsom salt, cornstarch and citric acid to a mixing bowl; whisk to combine.
In a small bowl, combine coconut oil, water, marshmallow oil and green soap colorant.
Add the oil mixture to the dry ingredients, one tablespoon at a time. It's important to add the oils slowly to avoid activating the bath bombs. Whisk the entire bath bomb mixture for about 3 minutes or until you have achieved your desired shade of green. The mixture is ready to mold if it clumps like damp sand (not wet) when you squeeze it in your hand. If it doesn’t clump to the desired texture, add an additional 1 teaspoon of water and whisk again.
Add a jumbo heart sprinkle to each mini muffin cup. Cover each heart with the bath bomb mixture and press down firmly with your fingers. Allow to dry overnight.
Carefully press the bottom of the mold to release each Grinch bath bomb from the mold.
Store the bath bombs in a sealed, airtight container. If you're giving these homemade bath bombs as holiday gifts, tie a festive ribbon around them and call it a day. No gift wrapping needed... the bright colors in a clear jar are too pretty to cover up!