Make the brownie batter:

Heat a medium Saucier or Saucepan over medium-low. Add butter and allow to melt slightly. Add chocolate and cook, stirring, until melted and smooth. Remove from heat.

Stir in granulated sugar until smooth. Add vanilla and stir to combine. Add eggs, one at a time, stirring to incorporate.

Add flour, cocoa powder, baking powder, and salt and stir until smooth. Set aside.

Make the blondie batter:

Heat a medium or large Saucier or Saucepan over medium-high. Add butter and cook, whisking. Continue cooking, whisking and watching closely, until foam subsides, and color changes from yellow to golden tan then quickly to a deep golden brown with dark brown milk solids at bottom of pan. Remove from heat and immediately add brown and granulated sugars, stirring to combine.

Whisk in egg until batter has a creamy consistency. Add flour, baking powder, and salt and whisk to combine. Set aside. 

Assemble and bake:

Preheat oven to 350F. Use a spoon or offset spatula to spread the brownie batter evenly in a Baking Slab.

Use the same spoon to place large dollops of blondie batter on top of brownie batter (it doesn’t have to be even yet).

Press batters into corners of Baking Slab and use the same spoon to swirl batters together in a zigzag pattern across the pan. Press into an even layer as best you can.

Bake until center feels slightly firm when touched and a cake tester or toothpick inserted into center comes out with some crumbs attached, not totally clean. Check after 25–30 minutes and return to oven for an additional 10 minutes if necessary.

Let cool for 10–15 minutes then cut, serve, and enjoy.