In a medium bowl, sift together flour, baking soda, and salt. Set aside.

In a large bowl, place the melted butter, granulated sugar, and brown sugar. Whisk until well combined. Whisk in the egg, egg yolk, and vanilla extract. Add flour mixture and mix just until combined; do not over mix. Fold in M&Ms.

Chill the dough for a minimum of 2 hours, preferably 6 hours or overnight (or up to 3 days).

If the cookie dough is hard to handle, leave it on the counter for a few minutes to soften.

Drop rounded balls of dough, around 3 tablespoons in size each, onto prepared baking sheet. Press extra M&Ms into the tops. Bake for 10-12 minutes, or until cookies begin to brown at the edges and the center is still soft. Cookies will appear undone but will continue to bake on the warm baking sheet. Leave to cool for 10 minutes, then transfer to a wire rack to cool completely.

Store cookies in an airtight container for up to 3 days.

Cookies can be frozen for up to 2 months.