I think the key to making this work is having cases that are sturdy enough to hold the melted chocolate. The ones I used are much thicker than the usual thin paper style cupcake cases.
Melt the butter and chocolate in a pan over a saucepan of simmering water. Once they are melted stir gently to ensure a smooth consistency. Tip a small amount of the melted chocolate into the cases to form a base. Then using a small spoon put a dollop of your favourite peanut butter in the middle of the case. Cover the peanut butter with melted chocolate and leave to set for an hour or two.
It's that simple!!