In medium mixing bowl, combine flour, oats, baking soda, and salt. In a small mixing bowl, combine M&M's, white chocolate, and sprinkles.

In a large mixing bowl, stir together melted butter and brown sugar until thoroughly combined. Add egg and yolk one at a time. Stir to combine after each addition.

Add the vanilla and stir. Add flour mixture and stir just until combined. Gently fold in M&M mixture. Cover the dough and place in the fridge for 1 hour.

Preheat oven to 325°.

Roll dough into 2-tablespoon balls and arrange on greased or parchment-lined baking sheet. Bake for 12-15 minutes.

The cookies will look slightly underdone but will continue to cook after removing them from oven. Place baking sheet on a cooling rack for 5 minutes, then transfer cookies to a wire rack to cool completely.

Repeat with remaining dough.