Preheat your oven to 350°F (175°C) and line a 9×13 inch baking pan with parchment paper for easy removal.

For the brownie layer: In a large saucepan, melt the butter over medium heat. Once melted, remove from heat and stir in the sugars, mixing until combined. Add the eggs one at a time, stirring well after each addition. Mix in the vanilla extract.

Combine dry ingredients in another bowl: whisk together the flour, cocoa powder, salt, and baking powder. Gradually add this dry mixture to the wet mixture until just combined. Be careful not to overmix; a few lumps are okay!

Spread the brownie batter evenly into the prepared baking pan and set aside.

For the cookie layer: In a large mixing bowl, cream together the softened butter and sugars until light and fluffy. Add the egg and vanilla extract, continuing to mix until smooth.

Mix the dry ingredients: In a separate bowl, combine the flour, baking soda, and salt. Gradually add this to the wet ingredients, mixing just until incorporated. Stir in the chocolate chips or any other mix-ins you prefer.

Drop spoonfuls of the cookie dough over the brownie layer, using the back of a spoon to gently spread it evenly.

Bake for 25-30 minutes, or until the edges look set and a toothpick inserted into the center comes out with a few moist crumbs.

Cool in the pan for at least 10 minutes before lifting out using the parchment paper. Let it cool completely on a wire rack before cutting into squares.