Combine all the dry ingredients in a bowl. Add the butter to a saucepan and bring to a boil, stirring constantly, to make brown butter. Allow the brown butter to chill and then mix it together with both kinds of sugar, the vanilla extract, and the eggs. Beat everything until nice and fluffy, gradually fold in the dry ingredients, and work everything into a smooth dough. Refrigerate the dough for 15 minutes, fold in both types of chocolate chips, and refrigerate the cookie dough overnight.
Melt the chocolate chips over a hot water bath and pour half of the melted chocolate into a 10-inch tart pan. Use a spoon to spread the chocolate over the bottom and sides of the dish and refrigerate for 25 minutes.
Whip the peanut butter, powdered sugar, and butter and spread the mixture over the chilled chocolate in the tart pan. Smooth everything out, pour the rest of the melted chocolate over the peanut butter filling, and freeze for 2 hours.
Melt the chocolate chips. Then stir in the vegetable oil, cocoa powder, sugar, vanilla extract, salt, and milk. Gradually fold in the eggs and then the flour and mix until you get a smooth batter.
Preheat the oven to 350°F. Spread the cookie dough over the bottom of a 12-inch springform pan and use a spoon to firmly press it down. Remove the peanut butter cup from the tart pan and place it face down on top of the cookie dough in the springform pan. Pour the brownie batter over the peanut butter cup until it is completely submerged. Transfer the springform pan to the oven and bake for 45 minutes. Let the cake cool before serving.