Line a 12-cup muffin tin with parchment paper cupcake liners. Alternatively, you can use silicone muffin cups.
Melt the peanut butter, butter, and cacao butter together in a medium saucepan over low heat, stirring until smooth. Stir in the sweetener and vanilla extract until well combined and no lumps remain.
Divide the mixture among the prepared muffin cups and refrigerate until firm, about 1 hour.
Set a heatproof bowl set over a pan of barely simmering water. Put the chocolate in the bowl and stir until melted and smooth. Drizzle about 1 teaspoon over each peanut butter cup and spread to the edges. Return the cups to the refrigerator until set, another 10 to 15 minutes.
Store in the refrigerator. Keeps for up to a week.