1.  In the bowl of a stand mixer fitted with a paddle attachment, cream butter, brown, and granulated sugar together on medium speed, beating until light and fluffy about 2-3 minutes. Add in eggs and vanilla and mix until incorporated on low speed. Turn the mixer up to medium-high and mix for 1 minute.

2.  Scrape down the sides of the bowl if needed. Add the cornstarch, baking soda, and salt. Mix for 30 seconds then slowly add the flour. Mix until fully incorporated.

3.  Add the chocolate chips, white chocolate chips, and 3/4 cup M&M's, and either beat for a few seconds on low speed or fold in by hand.

4.  Drop 2 tablespoons of the dough or roll it into balls onto a lined baking sheet. Top with more chocolate chips & M&M's if desired. Cover with plastic wrap and chill the dough for at least 1 hour or longer in the refrigerator. (Overnight or a couple of days works too.) Or chill for 20-25 minutes in the freezer.

5.  When ready to bake, preheat oven to 350 F. Spread cookie dough on a baking sheet lined with parchment paper, spaced 2 inches apart. Bake for 8-10 minutes until the edges are just golden. Make sure not to overbake, the cookies will set up as they cool.

6.  Remove from oven, let cool on baking sheet for 5-10 minutes, and then transfer to a wire rack to finish cooling. (Store in an airtight container.)