• Preheat oven to 350℉. Spray an 11" tart pan with cooking spray. Set aside.
• In a large bowl using an electric hand mixer (or bowl of a stand mixer with the paddle attachment) cream together the butter, brown sugar, and peanut butter until light and fluffy, about 2-3 minutes.

  ^{*The 2-3 minute beat time is important. You want this mixture to get light & fluffy which will help the texture of the baked cookie.}
  1/2 cup (1 stick) butter, 1/2 cup creamy peanut butter, 1 cup light brown sugar
• Add in the egg and vanilla and beat just until incorporated.

  ^{*Scrape down the sides of the bowl, as needed, with a rubber spatula to ensure everything gets combined together.}
  1 large egg, 2 teaspoons vanilla extract
• In a separate smaller bowl, whisk together the flour, baking soda, cornstarch, and salt.
  3/4 cup all-purpose flour, 1 teaspoon baking soda, 1 teaspoon cornstarch, ¼ teaspoon salt
• Add the flour mixture to the wet ingredients and beat on low speed just until combined.
• Add the oats, chocolate chips, and M&M's. Beat on low speed just until dough is combined.
  1½ cups quick oats, 1 cup mini chocolate chips, 1 cup mini m&m's
• Spread the cookie dough into the prepared tart pan and press evenly with your fingers.

  ^{* If wanted, sprinkle a few extra chocolate chips and m&m's on top and gently press down. This will make for a prettier presentation.}
• Bake for 16-20 minutes or until lightly golden brown.

  ^{* The edges will be lightly golden brown and the center should look cooked without looking like it is raw wet dough. It will cook more as it cools so you don't want to overcook it.}
• Remove the cookie from the oven and let it cool completely on a cooling rack.

Chocolate Buttercream Frosting (Optional)

• In a mixing bowl, using an electric hand mixer, beat the butter until light and fluffy. Add powdered sugar, cocoa powder, and heavy cream and beat again until combined and creamy.

  ^{* Use more or less powdered sugar and heavy cream depending on the desired consistency. If using a lower-fat milk than heavy cream, you won't need as much of it in order to get the right frosting texture. Start with 1 tablespoon and go from there.}
  1/2 cup (1 stick) butter, 1/4 cup cocoa powder, 1½ cups powdered sugar, 2-3 tablespoons heavy cream
• Place the frosting into a piping bag with a large 1A tip and pipe it onto the outside of the cookie cake. You can also place the frosting into a Ziploc bag, snip the bottom corner off, and pipe it onto the giant cookie. Or make it really easy and spread a layer of frosting over top the giant cookie!