For the brownie layer:Preheat oven to 375 degrees F. Generously grease the bottom and sides of a 3 quart, 9 x 13 inch baking pan.
Combine cocoa powder, flour, and salt in a medium bowl. Sift and set aside.
Combine butter and sugar in a stand mixer bowl. Beat until mixture comes together; it will look slightly gritty.Beat in eggs one at a time, scraping down bowl after each addition. Beat on medium speed until mixture is creamy and light yellow. Beat in vanilla.
Add flour mixture bit by bit, beating on low speed and scraping down sides of bowl until well blended.
Pour batter into prepared pan.
Bake for 20 minutes. Brownies will still be jiggly and just beginning to set in center. If it's still liquid, bake for about 5 more minutes. For the blondie layer:Prepare blondie batter while brownies are baking. If brownies finish baking before you are done, remove from oven and let sit while leaving oven on. Reduce heat to 350 degrees F.
Combine butter and brown sugar in stand mixer bowl.
Mix on low speed until smooth.Beat in eggs one at a time, scraping down bowl after each addition.
Add vanilla and beat on medium until mixture is light and fluffy.
Combine flour, baking powder, and salt in a large bowl.
Add bit by bit into stand mixer bowl, mixing well and scraping down sides of bowl as needed.Fold in chocolate chips and nuts last. You may want to do this by hand as the batter will be very thick.Carefully spoon the blondie batter evenly over the top of the brownies. It's best to ladle it on a little at a time instead of pouring on a big heap and trying to spread it as it will sink into the brownie batter. The brownie layer should be completely covered.
Bake in oven for 30 to 40 minutes, until blondie layer is browning on edges and has a dull finish, and a toothpick inserted in the center comes out clean.
Remove from pan and let cool on rack before serving.
