Step 1

Preheat oven to 350°F. Line two cookie sheet with silicone baking mats or parchment paper.

Step 2

Cream butter and sugar in the bowl of a stand mixer fitted with the paddle attachment (or a large bowl with a hand mixer). Cream until the mixture is fluffy, about one minute.

Step 3

Add the egg, egg yolk, and vanilla and mix until smooth, then mix in baking soda, cream of tartar, and salt. Add flour and mix until cookie dough forms. Stir in 1 1/2 cups of M&Ms.

Step 4

Scoop 2 tablespoon sized balls 2-inches apart on cookie sheet. Bake for 11-15 minutes, or until the bottoms are just starting get golden and the top is no longer glossy. Press a few more M&Ms on the top of each of the cookie as soon as they come out of the oven. Cool at least 10 minutes on cookie sheet before removing. Cool completely before frosting.

Step 5

Store in an airtight container for up to 3 days or freeze for up to one month.