Prepare the Brownies:

Preheat the oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment paper, leaving overhang for easy removal. Melt the butter and semi-sweet chocolate together in a microwave-safe bowl or over a double boiler. Stir until smooth. Add the granulated sugar, brown sugar, eggs, and vanilla extract, whisking until fully combined. Sift in the flour, cocoa powder, and salt, and gently fold into the mixture until no streaks remain. 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. Allow to cool completely.

Make the Cookie Dough Layer:

In a large mixing bowl, beat together the butter, brown sugar, and granulated sugar until light and fluffy. Mix in the milk and vanilla extract. Gradually add the heat-treated flour and beat until smooth. Stir in the mini chocolate chips. Spread the cookie dough evenly over the cooled brownies.

Add the Chocolate Topping:

Melt the chocolate chips and coconut oil (if using) in the microwave in 30-second intervals, stirring until smooth. Pour the melted chocolate over the cookie dough layer, spreading evenly with a spatula.

Chill and Serve:

Refrigerate the brownies for at least 1 hour to set the layers. Once firm, lift them out of the pan using the parchment paper, slice into squares, and serve.