Add cacao butter, cacao powder, salt, and sweetener to a small pot. Heat over low fire until cacao butter is melted. Stir well to combine. Remove from heat.
Line a pan with cupcake liners. Pour in 1-1 1/2 tsp of melted chocolate mixture into each liner. Gently swirl to bring chocolate slightly up the sides of the liner. Place pan in freezer for 5 minutes to harden.
Remove from freezer and add 1-2 tsp of peanut butter evenly on top of chocolate. Pour an additional 1-2 tsp of melted chocolate over peanut butter layer. Freeze for 10 minutes until set.