Cookie dough layer:
In the bowl of a stand mixer fitted with the paddle attachment, stir together flour, salt, brown sugar and granulated sugar. Pour in butter and vanilla extract. Mix until a dry dough forms. Add chocolate chips and mix on lowest speed (this is "Stir" on KitchenAid machines).
When chocolate chips are evenly dispersed, add whole milk 1 tablespoon at a time with the machine on low speed. Remove dough from mixer bowl and press about 2/3 into the bottom and up the sides of the lined bowl.
Wrap remaining portion of dough in plastic wrap and save for later use. Ideally, the cookie dough layer should be about ¼ -inch thick, but mine was more like ½ -inch in places. Do the best you can with it.
In a medium saucepan sprinkle gelatin over water. Let stand for 1 minute. Whisk in eggs and sugar. Stir in hot milk. Cook over medium heat, stirring constantly. Mixture will thicken after 5 minutes or so. When done, it should be thick enough to coat the back of a spoon.
Whip cream until stiff peaks form. Gently fold whipped cream into the chocolate mixture until well blended. Remove bowl lined with cookie dough from freezer and pour in mousse mixture. Smooth the top to even with a rubber spatula. Place in the freezer until firm, 4-6 hours or overnight.
Stir together butter, sugar and vanilla in bowl. Add egg; beat well with spoon. Stir together flour, cocoa, baking powder and salt; gradually add to egg mixture, beating until well blended. Spread batter thinly and evenly in prepared pan.
Remove mousse from freezer. Place a piece of parchment paper or plastic wrap on a work surface. Roll out the remaining portion of cookie dough on the paper to ¼-inch thickness. With the help of the paper, flip the dough over onto the mousse and trim to fit inside the bowl. Lightly brush the cookie dough with water and then turn the brownie layer onto the cookie dough layer.
Press brownie into cookie dough gently but firmly. Trim the edges of the brownie so that it fits inside the bowl and is flush with the bottom of the cookie dough bowl. Trim edges of cookie dough bowl if necessary to make it flush with the brownie. You will have a little leftover cookie dough to nibble on.
Cookie dough layer: