Preheat oven to 350 degrees.
Over medium heat, melt butter in a 10-12 inch cast iron skillet (or five 6-inch skillets) until bubbly and completely melted. Pour butter into a mixing bowl; beat in brown and white sugars. Add eggs and vanilla; beat.
Stir in chocolate chips. Evenly spread batter in skillet(s). Bake 20-25 minutes (15-18 minutes for 6-inch skillets) or until edges are lightly browned. Cool skillet on wire rack. Cut cookie into wedges and serve warm or cold with scoops of vanilla ice cream.