Melt the butter almost all the way, in a heatproof bowl then add the sugar and whisk it in. Return to the microwave for another 30 seconds then add the vanilla and salt. Whisk the mixture very well, for about 2 full minutes.
Add the cocoa and whisk well. The mixture will be loose at first then start to be firm as you whisk. Add the eggs, whisking for about 3 minutes.
Add the flour and chocolate, and use a rubber spatula to combine.
Remove the chocolate peanut butter layer from the freezer and out of the pan, keeping the parchment paper intact in the pan. Pour ⅔ of the brownie batter into the pan in an even layer.
Place the peanut butter block on top, then pour over the rest of the brownie batter. Smooth it out so it reaches the corners.
Bake for about 40-45 minutes, until the middle is puffing up.
Allow to cool fully before slicing. If you freeze them the layers will slice very cleanly and you’ll get clear layers.