Add the BTMS-50, cetyl alcohol, tallow, cocoa butter, and argan oil to a heat-safe container.
Melt gently using a double boiler until smooth.
Remove from heat and let cool slightly.
Stir in essential oils and preservative if using.
Pour into silicone molds. Let harden fully overnight.
After shampooing, rub the bar between your hands or directly onto wet hair. Focus on the mid-lengths and ends, leave on for 1-3 minutes, then rinse thoroughly.