All the ingredients can be stirred up in a single bowl, for a quick cleanup that's as divine as this skillet cookie's double hit of fudgy flavor.
Preheat oven to 350 degrees. In a large bowl, mix butter and sugar with a wooden spoon. Stir in egg and vanilla, then flour, cocoa, baking soda, and salt. Stir in chips.
Transfer batter to a 10-inch ovenproof nonstick skillet; smooth top. Bake until just set in center and pulling away from sides, 20 to 22 minutes. Let cool 10 minutes; loosen edges. Turn out onto a wire rack. Reinvert onto a plate, slice, and serve.