Preheat oven to 375 degrees F (190 degrees C). Spray the bottom and sides of a 9x13-inch baking dish with cooking spray.
Mix flour, brown sugar, peanut butter, butter, milk, egg, vanilla extract, salt, and baking soda together in a bowl. Fold chocolate chips into the batter. Pour batter into the prepared baking dish.
Bake in the preheated oven until lightly browned and a toothpick inserted in the middle comes out clean, 20 minutes.
