Cream butter and brown sugar in the bowl of a stand mixer fitted with the paddle attachment. Mix in egg and vanilla until smooth. Add pudding mix, baking soda, and salt. Mix until combined. Mix in flour, then stir in sprinkles and M&Ms.

Scoop 2 tablespoon balls of cookie dough onto a cookie sheet. Cover and chill for at least 30 minutes.

Preheat oven to 350°F. Line cookie sheets with parchment paper. Place chilled cookie dough balls 2” apart on cookie sheet. Bake 9-11 minutes or until the edges just start to turn brown.

Cool 5 minutes on cookie sheet before transferring to a rack to cool completely. Store in an airtight container.