For the Brownie Layer:

1. 1. Preheat your oven to 350°F (175°C). Grease and line a 9×9-inch baking pan with parchment paper for easy removal.
   2. In a medium bowl, whisk together the melted butter, granulated sugar, eggs, and vanilla extract until smooth and combined.
   3. Sift in the flour, cocoa powder, salt, and baking powder. Stir until just combined, but be careful not to overmix.
   4. Pour the brownie batter into the prepared baking pan and spread it out evenly. Set aside.
2. For the Blondie Layer:
   1. In a separate bowl, whisk together the melted butter, brown sugar, eggs, and vanilla extract until smooth.
   2. Stir in the flour, salt, and baking powder, mixing until fully incorporated.
   3. Gently spoon the blondie batter on top of the brownie layer, spreading it out evenly with a spatula.
   4. Bake for 30-35 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs (but not wet batter). The edges should be golden and slightly pulling away from the pan.
   5. Let the brownies and blondies cool in the pan for about 10 minutes, then transfer to a wire rack to cool completely before cutting into squares.