Preheat oven to 350°F
Line 2 cookie sheets with parchment paper.
In a small skillet, melt 10 tbsp unsalted butter.
Scrape with spatula continually, until the butter gets foamy on top and the butter smell gets deeper.
Cook until the milk solids begin to brown, 2-4 minutes.
Transfer to a bowl, add the remaining 4 tbsp of cold butter, and set aside to cool, 15 minutes.
In a pie dish, combine 1/4 c packed brown sugar with white sugar, rubbing with fingers to break up the brown sugar. Set aside.
In a large bowl, whisk together flour, baking soda and baking powder
Add remaining 1 3/4 c brown sugar and eggs with cooled melted butter, along with vanilla and salt.
Stir to combine until smooth.
Add this to Flour mixture and stir until completely mixed
Add in peanut butter chips and mini peanut butter cups
Press mixture into square cake dish, spreading around until it's even.
Sprinkle the brown sugar mixture over the top of the pan.
Bake for 15 minutes, spinning the tray 180° halfway through.
The pan may appear underdone, but you don't want it to overcook.
Let sit until cooled.