Preheat oven to 350 degrees.
Line a baking sheet with parchment paper or a silicone baking mat.
In a small bowl, stir together butter, brown sugar, egg yolk, and vanilla with a spoon or rubber spatula until well combined.
Add the flour, baking soda and salt, gently stirring until just incorporated, making sure not to over mix your dough.
Fold in about 3/4's of the M&Ms and sprinkles.
Scoop dough onto center of baking sheet and shape it into a 4 inch flattened circle.
Sprinkle remaining 1/4 of the M&Ms and sprinkles over top.
Bake 12-15 minutes or until edges just start to turn a golden brown, but center is still soft and puffy.
Remove from oven and let cookie sit on baking sheet for about 5-8 minutes. The residual heat from the pan will continue to cook and set the middle of the cookie for a soft, chewy center and crisp, buttery edge.
Transfer to a wire rack to cool completely.
Store in an airtight container for up to 4 days or freeze for up to one month.
Leave it out with a glass of milk for Santa on Christmas Eve! And remember to leave a snack for the reindeer.