Heat oven to 350ºF. Grease (spray) a 9 by 9 inch square brownie tin. Line it with parchment paper.
Make the Brownie: In a heatproof bowl, melt butter and chocolate in the microwave for about 2 minutes, stir using a whisk halfway through. Whisk until smooth.
Whisk in cocoa, set aside.In a separate bowl, whisk together eggs, sugar, vanilla and salt. Whisk in the cocoa mixture.
Using a spatula or wooden spoon, stir in the flour until just combined.
Pour into prepared pan, spread evenly.
Dollop the peanut butter all over.
Make the Cookie Top: Melt butter in a saucepan, take off heat, add sugars, whisk to combine well. Set aside to cool.
In a medium size bowl, combine flour, baking powder and salt.
Add egg to cooled butter/sugar mixture. Whisk a lot until the colour lightens a little. Add vanilla.
Add flour mixture to butter mixture in the pot. Stir with a rubber spatula until just combined. Add chocolate and nuts (if using).
Dollop the cookie batter all over the brownie top, almost like you are dropping cookies onto a sheet.
Bake for 35-45 or until lightly browned. Let cool completely before unleashing and devouring (if you can wait).