Preheat oven to 350°F.
Using a standing or hand mixer, cream the butter and sugars together in a large bowl, until fluffy and smooth.
Add egg and vanilla, beating until fully combined.
In a large bowl, add the flour, pudding mix, baking soda, and salt. Mix together and slowly add to the wet ingredients, just until combined.
Fold in the chocolate chips and M&Ms. Using a medium cookie scoop, scoop dough onto cookie sheets leaving 1 inch between each cookie.
Bake for 10 minutes and let cool on cookie sheets. Transfer to an air-tight container for storage.
