Preheat the oven to 350°F / 175°C. Grease and line a 8" square pan with butter and parchment paper.
Peanut Butter Cookie Dough
In a small bowl, melt the butter. Whisk in the peanut butter and brown sugar until combined, then whisk in the egg yolk.
Add in the flour, baking soda, baking powder, and salt. Fold the mixture together just until no flour streaks remain. Set aside.
Fudge Brownies
In a medium saucepan set over medium heat, melt together the butter and chocolate. Turn off the heat and immediately whisk in the cocoa powder. Set aside.
In a medium sized bowl with a hand mixer, beat together the eggs and sugar on medium high speed for 5 minutes, until the mixture is light and fluffy.
Pour the melted chocolate mixture into the egg mixture, and fold until combined.
Add in the flour, salt and chocolate chips if using. Fold the mixture together just until no flour streaks remain.
Pour the brownie batter into the prepared pan. Use your hands to tear off chunks of the peanut butter cookie dough, and place them on top of the brownie batter. Lightly push the cookie dough into the brownie batter.
Bake for 30-35 minutes, or until the brownies only slightly jiggle in the pan and the cookie dough is golden brown.
Wait for the brookies to cool to room temperature before slicing. Serve and enjoy!