1. Preheat oven to 350 degrees. In a mixer, beat the butter and brown sugar together on medium-high speed for about 2 minutes.
2. Add in the egg and vanilla, continue beating on medium-high speed until combined. Scrape down the sides of the bowl. Mix in the vanilla pudding, baking soda and salt. Mix until combined again.
3. Add in the flour one cup at a time, mixing after each cup. Mixture will be thick. Add in the M&Ms and beat on low or stir by hand.
4. On a baking sheet lined with a Silpat or parchment paper and using a large cookie scoop, scoop dough onto the sheet. Slightly flatten the balls of dough.
5. Add M&Ms to the top of the dough to garnish prior to baking. This adds extra candy, but also makes them look super pretty. Bake for 8-10 minutes. Remove baking sheet and cool.