3/4 cup (1.5 sticks) unsalted butter, softened to room temperature
1 cup packed light brown sugar
1/2 cup granulated sugar
1 large egg, at room temperature
1 large egg yolk, at room temperature
2 teaspoons pure vanilla extract
1 cup M&Ms
Whisk the flour, baking soda, and salt together in a large bowl. Set aside.
Throughout this step, continue to scrape the dough off of the sides of the bowl between stirs whenever necessary. In large bowl using a hand-held mixer or stand mixer fitted with a paddle attachment, beat softened butter for about 1 minute on medium speed until creamy. Add the brown sugar and granulated sugar, then beat on medium-high speed until creamy and combined.Beat in egg, egg yolk, and vanilla extract. With the mixer running on low speed, slowly add dry ingredients to wet ingredients. Pour in the M&Ms and beat on low speed for 20 seconds or until combined. Once completely combined, cover cookie dough tightly and chill in the refrigerator for 30 minutes and up to 5 days.
Preheat the oven to 325°F (163°C). Remove cookie dough from the refrigerator. In the middle of the baking tray, place the dough then spread it out evenly into a rectangular shape. Once again, I just did one large rectangle, rather than two separate ones, which makes these cookies thicker than usual. If you want to have thinner cookies, be sure to divide the dough in half and make the two rectangles. Bake for 8-10 minutes or until edges are slightly browned. The centers should appear very soft and puffy. If desired, press a few M&Ms into the tops of the warm cookies for looks. Repeat with remaining cookie dough. Enjoy!