Melt the fats: In a double boiler, gently melt the tallow and cocoa butter over very low heat until completely melted. To make a homemade double boiler, place a jar in a saucepan half-filled with water, with a folded tea towel on the bottom to keep the jar from sitting directly on the pan. Alternatively, place a metal bowl over a pot of gently simmering water.
Add the calendula oil: Remove the melted fats from the heat and stir in the calendula-infused oil until well combined.
Add essential oils: Let the mixture cool until just warm, then stir in the essential oils, if using. I added 15 drops each of lavender and frankincense essential oils.
Cool the mixture: If you don't want to whip your body butter, you can transfer it to a clean, dry jar at this point. Otherwise, let the mixture cool until it is thick and opaque but not solid, with a texture similar to softened butter. To speed up cooling, place the jar or bowl in a bowl of ice or put it in the refrigerator. Check frequently and stir as it cools to prevent the mixture from solidifying around the edges too quickly.
Whip or blend (optional): Once the mixture has thickened, whip it with a hand mixer for a lighter, fluffier texture. Alternatively, use an immersion blender for a smoother, creamier, lotion-like consistency, as I did.
Transfer to a jar: Use a spatula to transfer the finished calendula body butter to a clean, dry jar. Close the lid tightly and store it in a cool, dry place away from direct sunlight. If your home is very warm, you can keep it in the refrigerator to prevent it from becoming too soft.