• Preheat oven to 350℉.
• In a small microwave-safe bowl, melt unsalted butter (½ cup, i.e. 1 stick) for 20-30 seconds until JUST melted. 
• Add brown sugar (½ cup) and granulated sugar (¼ cup) and use a spatula to stir until combined (* do not use a mixer!). Add in egg (1 large) and vanilla extract (1 ½ teaspoons), then mix.
• Next, add in baking soda (½ teaspoon), baking powder (¼ teaspoon), and salt (¼ teaspoon) and mix again. Add in all-purpose flour (1 ½ cups) and mix until all the flour disappears into the dough.
• Add chocolate chips (½ cup) and M&M's (½ cup) into the dough and mix. Immediately proceed to pressing into the greased pan (you don't want the dough sitting out or it will set too much with the melted butter).
• Grease a 10.5" metal cookie cake pan (don't have one? See notes section.) Although this cookie removes cleanly, if you are worried about it sticking, line the bottom and sides of the pan with parchment paper first. Using a spoon or your clean hands, drop the dough into small chunks in the pan. This will help disperse everything so that you can easily smooth it out. Once all dough as been added, use a spatula or your hands to press down the dough into a smooth layer (being sure to get all the corners).

  Tip: If you notice 2-3 chocolate chips or M&M's are stuck together, separate them before baking. This will allow the chocolate to be evenly dispersed and prevent the cookie from having a giant clump of chocolate in one spot (which could cause it to more easily break while cutting).
• Bake at 350℉ degrees for 12-14 minutes. Do NOT overbake or your chocolate chip cookie cake will not be soft. You want the cookie cake to be slightly brown on the top and look slightly underbaked (my oven was perfect at 12 minutes). Remove from oven and place entire pan on a cooling rack. Allow the cookie cake to cool completely before removing.

  Tip: To remove from pan, use a knife to go around the edges, place a flat platter on top, then flip over the pan to remove. If you used parchment paper, this should come out rather smoothly. After flipping over the cookie cake, flip it over again on the final decorate plate or platter. Allow cake to fully cool before frosting.

To Make Frosting:

• Beat the softened butter (¼ cup, i.e. ½ stick) on medium-high speed for 2-minutes until smooth. Turn the mixer to low speed then slowly add the powdered sugar (1 ¼ cups). You may need to stop and scrape down sides. Add vanilla extract (1 teaspoon) and pinch of salt and turn mixer to medium speed.
• One tablespoon at a time, add milk (1-2 tablespoons total). Beat on high for 2-3 minutes until creamy and the frosting is at a good consistency to pipe. If it looks too runny, add more powdered sugar. If it looks too stiff, add a tiny bit of milk until you get to the desired consistency.
• This will make just enough frosting to pipe a border along the edges. If you would like more frosting to cover the top or to write words on top, simply double the frosting recipe!

Decorating:

• Using a large star piping tip, pipe around the edges then immediately add M&M's or sprinkles on top (the frosting will start hardening quick, so add sprinkles right away!).

  If you're feeling fancy, you can double the frosting recipe to have enough to pipe phrases like "Happy Birthday" on the top. You may need to add a little more milk to so that the frosting is more fluid to write with.