Preheat the oven to 350 degrees F. Lightly grease an 8-inch cast-iron or ovenproof skillet or 8x8-inch baking pan with nonstick spray.
In the bowl of a food processor fitted with a steel blade, place the chickpeas, oats, peanut butter, maple syrup, vanilla extract, cinnamon, baking powder, salt, and milk. Blend until mostly smooth. Remove the lid and scrape down the sides of the bowl, then blend for an additional 2 minutes until very well combined and smooth. The batter will be very sticky and thick. Fold in 1/3 cup chocolate chips.
Scrape the mixture into the prepared skillet and sprinkle the remaining 2 tablespoons chocolate chips over the top. If using a skillet, bake for 25 to 30 minutes until the top is dry to the touch and a toothpick inserted in the center comes out clean with just a few crumbs clinging to it. If using an 8x8-inch pan, begin checking for doneness at 20 minutes. Do not overbake or the cookie will taste dry. Place on a wire rack to cool. Slice and serve.
To Store. Leftover skillet cookie may be stored covered at room temperature for up to 3 days.
To Reheat. Warm the cookie in the microwave for 15-30 seconds for best results.
To Make Ahead. Mix the cookie dough and store it in an airtight container in the fridge for up to 24 hours and then bake as directed.