Preheat oven to 350F and line a 9x9 baking pan with parchment paper*
Combine butter and ½ cup (100g) semisweet chocolate chips in a large, microwave-safe bowl.
12 Tablespoons unsalted butter cut into Tbsp-sized pieces, ½ cup semisweet chocolate chips
Microwave for 30 seconds. Stir well, microwave for another 15 seconds and then stir well again. Repeat for 15 seconds until chocolate and butter are completely melted and well-combined.
Add cocoa powder and instant coffee (if using), stir well.
½ cup natural cocoa powder, ½ teaspoon instant coffee grounds optional
Add sugars, stir until completely combined.
¾ cup sugar, ¾ cup brown sugar packed
Add eggs, one at a time. After each addition, stir well, and then stir another 30 seconds (the better you stir your batter, the cracklier your brownie tops will be).
2 large eggs + 1 egg yolk
Stir in vanilla extract.
1 teaspoon vanilla extract
Sprinkle salt over batter and add flour, stir very well, until completely combined.
½ teaspoon salt, 1 cup all-purpose flour
Place 15 of your Oreos in a large Ziploc bag and use a rolling pin or mallet to smack until broken into pieces (don't crush into fine crumbs, you want decent sized pieces to bite into!).
20 Oreo Cookies
Add broken cookie pieces and mini chocolate chips and stir into batter.
⅓ cup mini chocolate chips
Spread batter into prepared 9x9 pan.
Break remaining 5 Oreo cookies into pieces and press gently into the surface of the brownie batter.
Bake on 350F (175C) for 30-35 minutes if using 9x9 pan (toothpick inserted in center should be just slightly fudgy, but not wet with batter).
Allow to cool before cutting and serving.
Notes
*Alternatively you may grease and flour your pan -- the parchment paper is a little wiggly when you're trying to spread your batter into it, but I prefer it for ease of cleanup.