Preheat the oven: Preheat your oven to 350°F (175°C) and grease an 8x8-inch baking pan or line it with parchment paper for easy removal.
Prepare the brownie layer: In a mixing bowl, combine the melted butter and granulated sugar. Stir well until the mixture is well combined.
Add the eggs one at a time, mixing well after each addition. Then mix in the vanilla extract.
In a separate bowl, sift together the cocoa powder, flour, salt, and baking powder. Gradually add the dry ingredients to the wet mixture, stirring just until combined. Be careful not to overmix.
Bake the brownies: Pour the brownie batter into the prepared baking pan and spread it evenly. Bake for 20-25 minutes, or until a toothpick inserted in the center comes out mostly clean (a few crumbs are okay).
Cool the brownies: Remove the brownies from the oven and let them cool in the pan for about 15-20 minutes.
Prepare the cookie dough layer: In a large mixing bowl, cream together the softened butter, brown sugar, and granulated sugar until the mixture is light and fluffy.
Mix in the vanilla extract. Gradually add the flour until just combined. The dough will be thick. Stir in the mini chocolate chips and milk until the desired consistency is reached (the dough should be soft but not sticky).
Spread the cookie dough layer: Once the brownie layer has cooled, evenly spread the cookie dough over the brownies. Use your hands or a spatula to gently flatten it.
Chill and cut: Place the assembled bars in the refrigerator for at least 30 minutes to set. After chilling, cut into squares or rectangles.
Notes
Serve the bars on clean trays lined with parchment paper and garnish with a sprinkle of sea salt for a sweet-salty kick or drizzle melted chocolate on top. Enjoy!