Preheat your oven to 350ºF and line two cookie sheets with parchment paper.
Cream the softened butter, white sugar and brown sugar together in the bowl of your stand mixer with the paddle attachment until light and fluffy. About 2 minutes.
Add in your egg, egg yolk and vanilla and cream on low until combined. Scrape the bowl to make sure everything is combined evenly.
Add in the flour, baking powder, baking soda and salt and mix on low until combined.
Scoop the dough onto your cookie sheet using a #20 scoop (about two tablespoons per scoop) and place a few M&M's on top so you can see the colors when they bake.
Bake for 10-12 minutes or until the center of the cookie is no longer shiny. Cookies will be pale.
Allow your cookies to cool on the pan for 5 minutes before transferring them to a wire rack to cool the rest of the way. Store at room temperature in an air tight container for up to two weeks.