Prepare the brownie layer:
Preheat your oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment paper.
Melt the butter and chopped chocolate in a microwave-safe bowl in 30-second intervals, stirring until smooth. Let it cool slightly.
Whisk in the granulated sugar, brown sugar, eggs, and vanilla until smooth.
Sift in the flour, cocoa powder, and salt. Gently fold until just combined.
Pour the batter into the prepared pan and bake for 25–30 minutes or until a toothpick inserted into the center comes out with a few moist crumbs. Let the brownies cool completely.
Make the cookie dough layer:
In a mixing bowl, beat the softened butter, brown sugar, and granulated sugar until creamy.
Add the milk and vanilla extract, beating until smooth.
Gradually mix in the heat-treated flour until combined, then fold in the mini chocolate chips.
Assemble the brownies:
Once the brownies are completely cool, spread the cookie dough layer evenly on top.
Optional ganache topping:
Heat the heavy cream until just simmering, then pour it over the chocolate chips. Let it sit for 1–2 minutes, then stir until smooth.
Pour the ganache over the cookie dough layer and spread evenly.
Chill and serve:
Refrigerate the brownies for at least 2 hours to set. Slice into squares and enjoy!