Combine baking soda, cornstarch, citric acid, and Epsom salts in a bowl. Whisk to remove any clumps.
In a separate bowl, mix water, essential oil, coconut oil, and food coloring.
Slowly pour the liquid mixture into the dry mixture, whisking as you go.
Once combined, take a small amount in your hand, and squeeze together. If the mixture is still powdery, gradually add water until it holds together. Fill a heart-shaped mold with mixture, or shape into a heart with your hands.
Allow to dry for 10-15 minutes. If using a mold, remove the mixture and allow to dry for 8 hours. Once dry, wrap in plastic or store in a dry place until ready to use.