1 1/2 sticks unsalted butter, softened

1 cup light brown sugar, lightly packed

1 large egg, room temp

1 TBSP vanilla extract

2 cups all-purpose flour

1 TBSP cornstarch

1 tsp baking soda

1/2 tsp kosher salt

11 oz package peanut butter m&m's

In the bowl of a stand mixer fitted with a paddle attachment (or large mixing bowl with an electric mixer) cream the butter, sugar, egg, and vanilla, beating on medium-high speed until light and fluffy. About 3 minutes.
Scrape down the sides of the bowl, and add the flour, cornstarch, baking soda, and salt. Beat on low speed until just combined. Scrape down the bowl as needed.
Add the M&Ms, and beat on low speed until just combined.
Use a large cookie scoop (or 1/4 cup measuring cup) to form large dough mounds. Place about 6 cookies each on an ungreased baking sheets, or sheets lined with parchment or a silpat. Use your hand to flatten the dough just slightly, keeping the cookie dough still very tall.
While the oven is preheating to 350F, place the cookie sheet in the freezer for about 5 minutes before baking. This will chill your dough and allow the outside to set before spreading too much while baking.
Bake at 350F for 11-12 minutes, or until edges have set and tops look set. The middle will still look undercooked. Cookies firm up as they cool.
Let cookies cool on the baking sheets for 10 minutes before serving warm or moving to a cooling rack to completely cool.