Preheat oven to 325 degrees. Lightly coat a 10-inch cast iron skillet with cooking spray. (Alternatively use 4 individual 6-inch cast iron skillets).
Beat brown sugar, white sugar and butter with an electric handle beater until light and fluffy. Add egg, milk and vanilla and beat until blended. In a seperate bowl whisk together flour, baking soda and salt. Add to wet mixture gradually beating at low speed until combined. Mix in 1 cup of chocolate chips.
Spread mixture evenly into the cast iron skillet(s) and top with remaining chocolate chips. Bake in preheated oven for about 45 minutes, until golden brown and center is just cooked through. (If using smaller individual skillets, bake 20-25 minutes). Remove from oven and let rest for 15 minutes before slicing and eating.