• Preheat the oven to 350 degrees F. Spray a 9x13 baking pan with nonstick spray or line with parchment paper. If using parchment paper, I spray the pan first and then press the parchment paper into the pan. The spray helps it adhere to the pan. Set aside for later. 

Brownie Layer

• In a small mixing bowl, whisk together the flour, cocoa powder, and baking powder until combined. Set aside. 
• In a separate large mixing bowl, whisk together the melted butter, sugar, and large eggs until well combined. 
• Dump the dry ingredients into the wet ingredients and mix with a rubber spatula just until combined, no flour streaks remain in the brownie batter, and a smooth & thick dough forms. 
• Stir in the chopped walnuts and chocolate chips until distributed. 
• Spread the brownie batter into an even layer into the bottom of the prepared baking pan.

  ^{* It's helpful to drop spoonfuls of it all over rather than in one big pile in the center.}

Chocolate Chip Cookie Layer

• In a small bowl, whisk together the flour and baking powder. Set aside. 
• In a separate large bowl, using an electric hand mixer, cream together the butter, granulated sugar, and light brown sugar until fluffy, combined, and lighter in color. About 1-2 minutes. 
• Add in the eggs and beat on low speed just until combined. 
• Add in the dry ingredients and mix on low speed just until combined.
• Stir in the chocolate chips until incorporated. 
• Crumble the cookie dough evenly across the top of the brownie batter.

Bake & Cool

• Bake for 28-33 minutes or until a toothpick inserted into the center of the brookies comes out gooey/soft but not wet batter. The brookie bars should not wobble when the pan is softly shaken. If they do, more than likely there is still some raw batter in there and they need to cook longer. If the top begins to brown too quickly then loosely lay a piece of foil over top the pan until they're finished baking. 
• Remove the pan from the oven and allow the brookies to cool completely at room temperature before cutting into squares and serving.