Step 1

Preheat oven to 350F.

Step 2

In a large mixing bowl, use a hand mixer or stand mixer cream together butter and sugars on medium speed until light and fluffy, about 2 minutes.

Step 3

Add egg and vanilla, and beat on low until incorporated.

Step 4

Add flour, baking soda, and salt and stir with a large wooden spoon until just combined. Fold in m&m's.

Step 5

Use a medium cookie scoop to scoop out 2 tablespoons of cookie dough. Drop onto a large half sheet baking tray lined with a silicone baking mat or parchment paper. For a cookie with smooth edges, use your hands to roll it into a ball. Space the cookies a few inches apart (you can fit about 6-8 on a large cookie sheet). Press in extra m&m's into the top of the cookies.

Step 6

Bake for 8-10 minutes, until edges just start to brown. DO NOT OVERBAKE. The cookies may not look done in the middle, but they will continue to cook as they are cooling. Remove from oven and allow the cookies to cool in the baking sheet for 5 minutes, then transfer to a wire cooling rack to cool completely.