How to Make It
Preheat oven to 325°F. Lightly coat a 10-inch cast-iron skillet with cooking spray. Beat brown sugar, granulated sugar, and butter with a heavy-duty electric stand mixer at medium speed until light and fluffy. Add egg, milk, and vanilla, beating until blended.
Whisk together flour, cocoa, baking soda, and salt in a bowl. Add to butter mixture gradually, beating at low speed until combined.
Add 1 cup of the chocolate chips; beat until combined.
Spread mixture evenly in prepared skillet. Top with remaining 1/2 cup chocolate chips.
Bake in preheated oven until golden and set, about 50 minutes, topping cookie with miniature marshmallows during the last 30 minutes of baking. Let stand 15 minutes; cut into wedges. Serve with vanilla ice cream.