Preheat the oven to 350°F/180°C. Grease an 8×8 pan and line it with parchment paper
In a small bowl, whisk together the flour and salt
In a large jug, whisk eggs, egg yolk, sugars, oil, espresso powder and vanilla extract
Over a double boiler, melt the butter and chocolate over low heat until it’s completely melted and smooth
Add the cocoa powder to the chocolate and whisk until smooth
Pour the sugar mixture into the chocolate mixture and whisk until smooth
Add the flour mixture and fold, making sure there are no air pockets
Fold in most of the chopped peanut butter cups, set aside a small handful that you'll sprinkle on top later
In a medium bowl, melt the peanut butter for 15-20 seconds in the microwave. Whisk together the peanut butter, powdered sugar, salt and vanilla until smooth
Pour the brownie batter into the prepared tin, then dollop the peanut butter filling on top. Use a butter knife to create swirls.
Sprinkle your reserved peanut butter cups on top. Bake for 25-30 minutes. The top should have crackled, and a toothpick inserted will have a few crumbs on it.
Transfer to a wire rack and let cool
Notes
Peanut butter: do not use natural peanut butter for this and instead use creamy, conventional peanut butter.Butter: I use European style butter in this recipeSalt: If you don’t have kosher salt, halve the amount of salt called for in the recipe. 1 teaspoon kosher salt = ½ teaspoon fine salt. Please note, this is diamond crystal salt and not Morton salt, which is much too salty for this. Storage: Keep them in an airtight container, if you keep them at room temperature they'll last about 3 days but in the fridge they'll last about 5 days.