Preheat oven to 350℉. Use convection if available. Spray a small cast iron skillet (I use an 8 inch skillet) or a small cake pan with nonstick cooking spray.
Thoroughly combine butter and brown sugar then mix in the egg yolk, vanilla, baking powder, and salt. Stir in flour. Finally, fold in chocolate chips.
Bake for about 20 minutes or until the outer edges and center are golden brown, but the center is still quite soft. The cookie will continue to bake when pulled from the oven and is best when still soft and gooey.
Allow to cool slightly. Best enjoyed while warm. Nice with ice cream!