To the bowl of a stand mixer fitted with the paddle attachment (or large mixing bowl and electric mixer) combine the butter, sugars, egg, vanilla extract, and beat on medium-high speed until creamed and well combined, about 4 minutes.
Stop, scrape down the sides of the bowl, and add the flour, pudding mix (or cornstarch), baking soda, optional salt, and beat on low speed until just combined, about 1 minute.
Stop, scrape down the sides of the bowl, and add the chocolate chips, 3/4 cup M&M’s, and beat on low speed until just combined, about 30 seconds.
Using a large cookie scoop, 1/4-cup measure, or your hands, form 15 equal-sized mounds of dough, roll into balls, and flatten slightly.
Add about 1 tablespoon M&M’s to the top of each dough mound.
Place mounds on a large plate or tray, cover with plastic wrap, and refrigerate for at least 2 hours, up to 5 days. Do not bake with unchilled dough because cookies will bake thinner, flatter, and be more prone to spreading.
Preheat oven to 350F, line a baking sheet with a Silpat or spray with cooking spray. Place dough mounds on baking sheet, spaced at least 2 inches apart (I bake 8 cookies per sheet).
Bake for 11 minutes, or until edges have set and tops are just set, even if slightly undercooked, pale, and glossy in the center. Cookies firm up as they cool.
Allow cookies to cool on baking sheet for about 10 minutes before serving. I let them cool on the baking sheet and don’t use a rack.
