Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.

In a large mixing bowl, combine the softened butter and sugar, beating until light and fluffy for about 2-3 minutes.

Add the egg yolks and vanilla extract, mixing until completely combined, about 1-2 minutes.

Gradually add the flour, mixing on low speed until just combined, ensuring not to over-mix.

Scoop out portions of dough and roll into balls about one tablespoon in size, spaced at least 2 inches apart on the baking sheet.

Roll each ball in sanding sugar for a sparkling, crunchy finish.

Gently press a chocolate M&M into the center of each cookie dough ball.

Bake in the preheated oven for 10 minutes until lightly golden around the edges.

Remove from the oven, cool on the sheet for about 3-5 minutes, then transfer to a wire rack to cool completely.