Preheat oven to 350 and make sure your 10-inch skillet is well seasoned. (You can also rub some oil with a paper towel so that the bottom and sides of the skillet are shiny but not drenched in oil.)
In the bowl of an electric mixer, cream together butter and sugars or a couple of minutes.
Add egg and beat until mixed. Scrape sides and bottom of the bowl.
Sift in the flour, baking soda and salt. Mix the ingredients on low speed until just combined.
Scrape vanilla bean and add seeds to the batter. Mix just until incorporated.
Divide batter in half and set half of vanilla batter aside in the skillet.
Add cocoa powder and melted chocolate to the leftover half of batter in the mixer and mix just until batter is evenly chocolate.
Spread vanilla cookie batter on half of the skillet and chocolate batter on the other half. using a thin spatula, swirl cookie batter together to form marble texture. Pat it down evenly.