Line a standard 12-cup muffin tin with paper liners.
Melt chocolate either in a double boiler.
With a small spoon or cookie scoop, evenly distribute half of the melted chocolate into each muffin cup. Drop pan repeatedly on the counter to help chocolate flatten and smooth out. Freeze whole pan for 15 minutes.
Meanwhile, in a medium bowl combine peanut butter, powdered sugar and butter. Whip with an electric hand mixer until smooth*. Remove pan from freezer and place small spoonfuls of peanut butter mixture on top of each chocolate layer. Drop pan repeatedly on the counter again, to help flatten peanut butter layer.
Portion small spoonfuls of chocolate into each cups, three cups at a time, immediately dropping the pan repeatedly on the counter to flatten cups*. Freeze whole pan for 15 minutes to set the top layer of chocolate.
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 up to 5 days.