Line a 9x9 inch baking pan (23x23 cm) with plastic film, leaving an overhang on the sides to easily lift it out before cutting and serving. Set aside.
In a large heat-proof bowl, add peanut butter and butter and microwave until completely melted, about 2 minutes. Stir every 30 seconds. Stir in powdered sugar until fully dissolved and combined. Transfer the half to the prepared baking pan. Chill 15 minutes in the fridge until the top is set to touch.
Add melted chocolate and melted butter to the remaining peanut butter mixture and stir to combine. Pour on top of the chilled peanut butter layer. Cover with plastic wrap and chill at least 4 hours preferably overnight. Before serving, cut into pieces. I cut 9x9 stripes and got 81 pieces. Store leftovers in an airtight container in the fridge up to 1 week.
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Notes
You could cut the recipe in half or fourth and pour it into a smaller pan (loaf pan, e.g.) if you don't plan to finish it within a week.