Line a standard 12-cup muffin pan with parchment paper cupcake liners or silicone liners.
Melt the peanut butter, butter, and cacao butter 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, filling each one-quarter full. Refrigerate until firm, about 1 hour.
Set a heatproof bowl over a pan of barely simmering water. Put the chocolate in the bowl and stir until melted and smooth. Drizzle about 1 teaspoon of the melted chocolate over each peanut butter cup and spread it to the edges. Return the cups to the refrigerator until set, another 10-15 minutes.
Store in the refrigerator for up to a week (if they last that long!).