Preheat oven to 400ºF. Line two baking sheets with parchment paper or silicone baking mats. Set aside.
In the bowl of a stand mixer with paddle attachment, add cubed butter, brown sugar and granulated sugar.
1 cup unsalted butter, ¾ cup dark brown sugar, ½ cup granulated sugar
Turn mixer on low for a few seconds, then gradually increase speed to medium-high, mixing for about 3-4 minutes until well incorporated and fluffy. Once the mixture begins sticking to the sides of the bowl, stop the mixer.
Turn mixer to low and add egg and egg yolk then vanilla. Mix on medium for about 30 seconds until well incorporated, stopping to scrape down the sides and bottom with a rubber spatula, as needed.
1 large egg, 1 large egg yolk, 1 ½ tablespoons vanilla extract
With mixer off, add in all-purpose flour, cake flour, baking powder, baking soda, and salt. Turn mixer on low and stir until just combined. Do not over mix.
1 cup cake flour, 1 ½ cups all-purpose flour, 1 teaspoon baking powder, 1 teaspoon baking soda, 1 teaspoon kosher salt
Add in M&Ms and chocolate chips and stir on low until evenly distributed.
2 cups M&M chocolate candies, 1 ½ cups semi-sweet chocolate chips
Using a large cookie scoop, portion dough into 2 ounce (¼ cup) size balls, rolling between your palms to ensure a smooth ball, if desired. Place about 2 inches apart on prepared baking sheets. Add additional M&Ms to the tops of each cookie ball
Bake in the preheated oven for 5 minutes then rotate baking trays and bake for another 5 minutes. Remove from the oven and use a 3-inch round cookie cutter to circle around the cookies, bouncing the sides of the cookie off the sides of the cookie cutter to form perfectly round cookies (optional).
Allow to cool for 10 minutes on the baking trays then transfer to wire racks.