Preheat oven to 325°F Use a 13×9 inch metal baking pan. Put a long piece of parchment paper in the bottom of the pan, letting the parchment extend up two sides of the pan and overhang slightly on both ends. (This will make it easy to remove the bars from the pan after they have baked.).
Place chocolate in a 1 cup glass measuring cup. Microwave for 1 minute and stir. Continue to microwave in 30 second increments until chocolate has completely melted; set aside to cool slightly.
In the bowl of a stand mixer fitted with the paddle attachment, beat together butter and brown sugar until combined, about 2 minutes. Beat in eggs and vanilla extract until combined, about 30 seconds. Mix in melted chocolate until combined. Mix in flour mixture until just incorporated.
Scrape batter into prepared pan and bake 25 to 35 minutes, or until toothpick inserted into the center comes out with just a few moist crumbs attached. Transfer pan to wire rack and cool completely.
To make the cookie dough layer, In the bowl of a stand mixer fitted with the paddle attachment, beat together butter, brown sugar and white sugar until light and fluffy, about 3 minutes. Beat in milk and vanilla until combined, about 1 minute. Mix in flour just until combined. Stir in chocolate chips.
Spread cookie dough over the cooled brownies.
To make the chocolate ganache, heat semisweet chocolate and milk in the microwave for 30 seconds then stir. Continue to microwave in 15 second increments until smooth. Pour over cookie dough layer. Transfer pan to the refrigerator to let chocolate ganache set up completely.