Step 1: Preheat your oven to 350°F (175°C). Line a 9x13 inch baking pan with parchment paper, leaving an overhang on the sides to easily lift the brownies out later. Grease the parchment.
Step 2: In a large microwave-safe bowl, combine 1 cup (2 sticks) unsalted butter, 1 cup granulated sugar, and 1/2 cup packed light brown sugar. Microwave in 30-second intervals, stirring after each, until the butter is melted and the sugars are dissolved. Alternatively, melt over a double boiler.
Step 3: Whisk in 4 large eggs one at a time, ensuring each is fully incorporated before adding the next. Stir in 2 teaspoons vanilla extract.
Step 4: In a separate bowl, whisk together 1 cup all-purpose flour, 3/4 cup unsweetened cocoa powder, 1 teaspoon salt, and 1/2 teaspoon baking powder.
Step 5: Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Be careful not to overmix; a few streaks of flour are okay. Stir in 1 cup chocolate chips or chunks.
Step 6: Pour the brownie batter into the prepared pan and spread evenly. Bake for 25-30 minutes, or until a toothpick inserted into the center comes out with moist crumbs, not wet batter. Do not overbake! Allow to cool completely in the pan on a wire rack.
Step 7: For the edible cookie dough, in a medium bowl, cream together 1/2 cup (1 stick) softened unsalted butter, 1/2 cup granulated sugar, and 3/4 cup packed light brown sugar until light and fluffy.
Step 8: Stir in 1 teaspoon vanilla extract and 1/4 teaspoon salt.
Step 9: Gradually add 1 1/2 cups heat-treated all-purpose flour. To heat-treat flour, spread it on a baking sheet and bake at 300°F (150°C) for 5 minutes, then let it cool completely. Mix until just combined.
Step 10: Fold in 1/2 cup milk (any kind works) until the cookie dough comes together and is smooth. Then, gently fold in 1 cup mini chocolate chips or regular chocolate chips.
Step 11: Once the brownies are completely cool, spread the edible cookie dough evenly over the top. You might find it easier to dollop spoonfuls of dough over the brownies and then gently spread it with an offset spatula or the back of a spoon.
Step 12: Chill the cookie dough brownies in the refrigerator for at least 2 hours before slicing and serving. This allows the cookie dough to firm up and makes for cleaner cuts. Store leftovers in an airtight container in the refrigerator for up to 5 days.