In a large mixing bowl, beat brown sugar, sugar, and butter until fluffy.
Add egg, milk, and vanilla, beating until combined.
In a medium bowl, whisk together flour, baking soda, and salt.
Add to mixing bowl gradually, beating until combined.
Fold in mini chips, chocolate chunks, peanut butter chips, and Reese's pieces.
Spray a 10-inch cast-iron skillet with non-stick spray.
Spread half the batter into the skillet.
Lightly press the Reese's cups into the batter in the skillet.
Spread with remaining batter.
Bake at 325 for 40-45 minutes, or until edges are golden and a toothpick inserted near the center comes out clean. You may have to poke it in a few places to make sure it's set in the center, since there's a lot of chocolate and chips in the cookie.
Cool for 15 minutes. Cut into slices.
Serve warm with ice cream and chocolate sauce.