Preheat oven to 350 degrees. Line a baking sheet with parchment paper or a silicone baking mat, set aside.
In the bowl of a stand mixer or a large mixing bowl with a hand mixer, cream the butter on medium speed for 1 minute. Add the sugars, mix on medium speed for 2 minutes. Add the egg and vanilla, mix until combined.
In a medium mixing bowl whisk together the flour, baking soda, and salt.
With mixing speed on low, gradually add the dry ingredients until a few streaks remain. Fold in M&M's and sprinkles with a spatula.
Using a standard size cookie scoop, portion dough and roll into smooth balls. Place on prepared cookie sheet.
Bake for 10-12 minutes or until the edges turn golden brown. Remove from the oven and let rest on the sheet for 3 minutes before transferring to a wire rack to cool completely.
Notes
-If you want a prettier appearance, reserve 2 tbsp. of M&M's for the top as well as 1 tbsp. sprinkles. Press into the the balls of cookie dough after they are scooped/rolled.