Preheat the oven to 350ºF. Spray a cast-iron skillet with coconut oil spray or baking spray. Set aside.
In a bowl, cream the softened butter and both sugars together for 1 minute until light in color and creamy. Add in the egg and vanilla and mix on medium for 1 minute until fully incorporated.
Stir in the peanut powder, flours, baking soda and salt. Mix for 1 minute. Scrape down the sides of the bowl once and mix for another 10 seconds.
Mix in the mini peanut butter cups and/or chocolate chips.
Add all the dough into the prepared skillet. Use a piece of plastic wrap to press the dough lightly into the skillet to flatten and even out—they don't have to be perfect—a little craggy will work! Then pop 'em in the oven.
Bake for 20 minutes or until the sides and top of the cookie have just started to brown. Remove from the oven and let cool for 20 minutes before cutting.