I tinkered with a few different recipes, but liked the outcome from the following one the best.
Added bonus: it's really simple!
Oh! This is super-important -- If you've never tried bath melts, they make your tub slippery. Unless you actually want the person getting in the shower after you to suffer a concussion, one of the caveats of using oils/butters in the bath is that you have to clean the tub afterward. Not so relaxing, I know, but it's worth it.
Lemon Lavender Bath Melts
2 oz. cocoa butter
2 oz. shea butter
1/2 teaspoon beeswax or carnauba wax (vegan)
1 teaspoon dried lavender buds (optional)*
12 drops lavender essential oil
12 drops lemon essential oil
(I also added 1/2 tsp of Vaniglia de Madagascar fragrance oil, because it makes everything smell amazing.)
*In this particular recipe, I used dried lavender buds, but I think I'll leave the lavender buds out, from now on. I didn't care for them floating around the bathtub, and clinging to me when I got out.
First off, it really helps to have a digital scale for recipes like this. I picked mine up at the post office, for around $30.
Set a large Pyrex measuring cup on top of the scale, and hit "tare" (this sets the scale to zero). Add the cocoa and shea butters til the scale indicates 2 oz each, then the beeswax. Set inside a pan/pot with an inch or two of boiling water (don't let any water splash into the pyrex cup), and stir until everything is fully melted.
Remove from heat, let cool for a few minutes (but before it starts setting), and add the lavender buds, and essential oils.
Pour into molds - I used a flower-shaped ice cube mold from IKEA, but you can use whatever small mold you have on hand, including candy molds, or a regular ice cube tray.
Let set for an hour or so (or pop into the freezer for 15 minutes). Pop out of molds, and store in a cool location until ready for use.
Tip: I used 2 melts when I initially tried these out, thinking more was better, but honestly, one should suffice.
And... please be careful when getting out of the tub, as it can be quite slick.