Prepare a 9x9 square baking dish or line it with parchment paper hanging over the sides by about 2 inches so it doesn't stick. In a large mixing bowl or the bowl of a stand mixer, mix together the peanut butter, soft room temp. butter, and vanilla extract with a whisk attachment until smooth.
Beat in the powdered sugar 1/2 cup at a time, until fully incorporated. Scrape down the sides of the bowl after each addition. The mixture should resemble a thick dough.
Turn the mixer off and transfer the peanut butter mixture to your prepared baking dish. Use a piece of wax or parchment on your hand to gently press the mixture into the bottom of the baking dish in an even layer.
Microwave the dark chocolate chips in a microwave bowl with coconut oil 20 seconds at a time, stirring between intervals, until melted and smooth. Pour the melted chocolate into the baking dish over the peanut butter layer and smooth it out with a spatula or spoon.
Sprinkle the top with a bit of flaky sea salt, then cover and put dish into the fridge to chill until set.
When firmed and set lift out by grabbing sides of parchment. Lay flat on counter and use sharp knife to cut into 24 equal pieces.