Preheat oven to 350 degrees. Line a baking sheet with parchment paper or a silicone baking mat. Set aside.

In a mixing bowl, use a hand-held mixer to beat together the butter and sugar until light and fluffy.

Add in the egg yolks and vanilla extract. Beat to combine.

Add the flour and mix into the butter mixture until just combined. Don't over mix.

Portion into one teaspoon and roll into balls. Roll each ball into the sanding sugar and place on the prepared baking sheet.

Push a M&M candy into the center of the cookie.

Bake for 10 minutes. Remove from oven and allow cookie to cool on the baking sheet for 3-5 minutes. Transfer the cookies to a wire cooling rack to finish cooling.