PREP: Preheat the oven to 350 degrees F. Lightly grease or spray a 9-inch cast iron skillet or 9- inch cake pan. Set aside.
CREAM BUTTER and SUGARS: In a medium-sized bowl, cream the room-temperature butter (See Note 1), white sugar, and brown sugar for about 2-3 minutes. Add in the egg and vanilla. Mix until combined.
DRY INGREDIENTS: In a separate bowl, stir together the flour (See Note 2), baking soda, baking powder, and salt.
COMBINE: Add the dry ingredients to the wet and mix until a dough forms. Do not over mix.
DIVIDE DOUGH: Divide the dough in half. Press one half of the dough into the bottom of the skillet or cake pan. Press the unwrapped and halved Reese's cups on top of that layer of cookie dough.
REMAINING DOUGH: To the remaining cookie dough, add in the milk chocolate chips and peanut butter M&M's. Stir to combine and then press that remaining cookie dough on top of the Reese's cups and smooth the dough into 1 even layer. The Reese's cups should be covered with dough.
BAKE: Bake for 20-25 minutes or until the edges are light golden brown. Remember, the center should be a little softer -- this is the hallmark of a delicious Pizookie!
SERVE: Serve immediately with ice cream and hot fudge if desired.