Preheat your oven to 350°F (175°C).
In a large bowl, cream together the softened butter, granulated sugar, and brown sugar for the brownies until light and fluffy.
Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
In a separate bowl, sift together the flour, cocoa powder, baking powder, and salt.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Fold in the semi-sweet chocolate chips.
Pour the brownie batter into a greased 9x13 inch baking pan and smooth the top.
Bake the brownie layer in the oven for about 25–30 minutes, until a toothpick inserted comes out with a few moist crumbs.
While the brownies are baking, prepare the cookie dough by creaming together the softened butter, granulated sugar, and brown sugar in another bowl.
Add the vanilla and eggs, mixing until well combined.
In another bowl, whisk together the flour and salt before gradually incorporating it into the wet mixture.
Fold in the mini chocolate chips to the cookie dough mixture.
Once the brownie layer is baked, remove it from the oven and let it cool for about 10-15 minutes.
Spread the cookie dough evenly over the cooled brownie layer.
Return the pan to the oven and bake for another 15-20 minutes, or until the cookie dough is lightly golden brown.
Once baked, remove from the oven and let cool completely before cutting into squares.
