Line your 9x9 inch pan with parchment paper, leaving some overhang for easy removal.
Melt the butter in a microwave-safe bowl for about 30 seconds until just melted and fragrant. Let it cool slightly.
In a mixing bowl, combine the creamy peanut butter with the melted butter, stirring until the mixture becomes smooth and luscious with a rich aroma.
Add the powdered sugar to the peanut butter mixture and stir thoroughly until fully incorporated. The mixture will thicken and become sticky but spreadable.
Press this peanut butter mixture evenly into the prepared pan, using a spatula or the back of a spoon to smooth the surface. Chill in the fridge for about 15 minutes to set.
While the peanut butter layer chills, melt the dark chocolate with the vanilla extract in a microwave-safe bowl, heating in 20-second bursts and stirring until glossy and smooth.
Pour the melted chocolate over the chilled peanut butter layer, spreading it out evenly with a spatula. Sprinkle a tiny pinch of flaky sea salt on top if you like that salty-sweet contrast.
Refrigerate the entire pan for at least 1 hour, or until the chocolate is firm and the bars are set. For faster results, pop it in the freezer for 30 minutes.
Once set, lift the bars out of the pan using the parchment overhang. Slice into 12 even squares with a sharp knife, wiping the blade between cuts for clean slices.
Serve immediately or store in an airtight container in the fridge for up to a week. Let sit at room temperature for 5 minutes before enjoying for optimal texture and flavor.
