Line a 9x13 baking pan with heavy-duty foil and very lightly grease the foil with vegetable shortening, like crisco or line with parchment paper.
In a small saucepan, melt the butter over your lowest setting, then add the unsweetened chocolate with the bittersweet chocolate chips stirring constantly with a small whisk until melted and smooth.
Turn off heat and let set on burner.
In a large mixing bowl, add the eggs into the bowl and lightly whisk.
In another bowl, add in the sugars and salt and whisk together, then add the eggs in and mix together until just incorporated.
Whisk in the melted chocolate into the sugar/egg mixture, until just combined.
Briefly whisk in the vanilla.
In another bowl, whisk the flour and baking powder together and sift into the chocolate mixture.
Stir with a silicone spatula and pour and spread out 2/3 of batter evenly into your prepared baking pan.
Place (1 bag) of peanut butter cups evenly throughout the brownies by pushing down into brownies evenly. Spread out the rest of the brownie batter on top. (It's ok if some of the peanut butter cups show).
Sprinkle milk chocolate chips on top of the brownie batter.
Bake 35 minutes, until toothpick comes out clean and transfer pan to cooling rack and let cool for 15 minutes.
Scatter roughly chopped peanut butter cups on top and place brownies in fridge to let brownies set.
While brownies are cooling, prepare the peanut butter chocolate drizzle.
Melt peanut butter and white chocolate together in microwave for a minute and whisk briskly to melt evenly. Place back in microwave 15 second increments, stirring each time, if further melting is needed.
Place peanut butter chocolate in ziplock bag and snip corn and drizzle it back and forth across brownies.
Once brownies are set, cut and enjoy! Best served at room temperature.
Can be refrigerated up to 3 weeks. Makes 12 very large or 24 small brownies.