Add the chocolate to a microwave-safe bowl and heat in 30 second intervals, stirring after each one, until melted and smooth.
Line a muffin pan with muffin cases, then add 1 tablespoon of melted chocolate into each case. Spread out evenly. Freeze until firm - about 10 minutes.
Add the peanut butter, vanilla, icing sugar, and butter to a microwave-safe bowl, and mix until combined. Heat until slightly runny, and can easily be poured.
Add 1 tablespoon of the peanut butter mixture into each muffin case, topping the chocolate layer. Freeze until firm.
Top the peanut butter layer with the remaining melted chocolate, and freeze until firm.
For a peanut butter cup with a firmer texture, serve chilled. For a softer, creamier texture, serve at room temperature.
Store refrigerated in an airtight container for up to 5 days.