Grease an 8-inch cast iron skillet with butter. Set aside.

In a large bowl, cream together butter and brown sugar until light and fluffy. Beat in 1/4 cup peanut butter and vanilla extract. Whisk in the egg until well blended.

Stir in the oats, flour, baking powder, baking soda, and salt. Fold in chocolate chips and M&Ms.

Press about ⅓ of the dough into the bottom of prepared skillet (the entire bottom and sides of the skillet should be covered with dough). Fill with 1/3 cup peanut butter. Top with more dough until the top is completely covered and the peanut butter is sealed inside. Garnish with extra chocolate chips and M&Ms if desired. Cover with foil and refrigerate for at least 30 minutes, or up to 3 days.

When you’re ready to bake, preheat oven to 350ºF.

Bake for 25 to 30 minutes or until golden brown on top. Serve immediately with ice cream.