Whisk melted butter and sugar together. Whisk in eggs one at a time. Whisk in vanilla.
In another bowl combine flour, baking soda and salt.
Slowly whisk dry ingredients into wet ingredients. Whisk in buttermilk.
Fold in cookie butter and chocolate chips.
Spread batter into a 10 inch cast iron skillet. Bake in preheated oven 30 - 35 minutes or until a toothpick inserted comes out clean. Allow to cool, slice and serve.