You’ll need a wide mouth 1 quart/32 oz. mason jar. Add the flour, cornstarch, baking soda, baking powder, and salt to the bottom on the mason jar first. (You can place them on a paper plate and fold the paper plate to easily get the ingredients into the jar if you don’t have a funnel, or just spoon it in). Tap on the counter to level it off.
Spoon in and layer the brown sugar in next, followed by the granulated sugar. Then, the semi sweet chocolate chips, and the M&M's® on top.
Preheat the oven to 350°F and line 2 baking sheets with parchment paper.
Add the butter, egg, and vanilla to a stand mixer bowl fitted with the paddle attachment. Combine for 2-3 minutes, everything won’t look very incorporated at this point and it’s OK.
Add all contents of the jar in and mix together on low speed, being careful not to break up the M&M's® too much. You could always pour the M&M's® out first and then stir them in to ensure they don’t break.
Scoop into ~2 tbsp. Sized cookies, and place on prepared cookie sheet.
Bake for 8-10 minutes. The cookies will look underbaked but set up as they cool. Swirl a cup or something a little larger around the cookies right out of the oven to give them a perfect circular look. Add more M&M's® or chocolate chips on top, if you have any!
Place on cooling rack, and enjoy! Store at room temperature in an airtight container for 3-5 days or freeze for up to 3 months.