The Preparation
Preheat oven to 325°F. Line an 8x8-inch pan with parchment paper and grease well.
Melt butter in a double boiler over simmering water.
Add cocoa powder, sugar, and salt to the melted butter. Stir until combined.
Remove from heat and allow to cool somewhat.
Add cold eggs one at a time, mixing well after each addition.
Add vanilla extract and stir.
Fold in flour until just incorporated — do not overmix.
Pour approximately half the batter into the prepared pan and spread evenly.
Press 9 unwrapped peanut butter cups in a 3x3 grid into the batter.
Pour remaining batter over the peanut butter cups and spread to cover.
Bake for 30-35 minutes until edges pull away and a toothpick comes out with moist crumbs.
Cool completely before cutting.
