Using stand mixer with paddle attachment, mix together butter, sugars, egg, and vanilla and beat until combined, about 1-2 minutes.
Scrape the bowl and add flour, pudding mix, and baking soda and mix until just combined, about 1 minute.
Add chocolate chips and mix for 30 seconds. Add 1 cup M&Ms and mix for about 15 seconds or until just mixed in, OR mix by hand so they don’t break.
Using 3 Tablespoon cookie scoop, scoop 15 heaping scoops (or ¼ cup scoop, fairly even scoops) and place on parchment lined baking tray. Evenly divide remaining M&Ms and press into top of each cookie dough scoop.
Refrigerate for 2 hours (or more).
Preheat oven to 350F. Line two baking trays with parchment paper and add cookie dough spaced evenly on the sheet (allow 2 inches between cookies).
Bake for 11-13 minutes, or until edges are just cooked/ browned. They may appear undercooked in the middle. Cool for 10 minutes or longer on baking tray and transfer to cooling rack.
Notes
Store cookies for up to one week at room temperature (if they last that long) or freeze for up to six months.Unbaked dough may be frozen. Defrost for one hour and bake as directed.