Prepare the dry ingredients: In a mixing bowl, whisk together the flour, baking soda, corn starch, and salt. Set aside.

Cream the butter and sugars: In the bowl of a stand mixer fitted with the paddle attachment, add the softened butter, brown sugar, and granulated sugar. Beat on medium speed until light and fluffy, about 3 minutes.

Add the eggs and vanilla: Add the eggs and vanilla extract to the butter mixture. Beat on medium speed until well combined, scraping down the sides of the bowl as needed.

Mix in the dry ingredients: Slowly add the dry ingredients to the wet ingredients. Mix on low speed until just combined.

Fold in the chocolate: Add the dark chocolate chips, regular M&Ms, and mini M&Ms. Mix on low speed until evenly distributed.

Chill the dough: Cover the dough with plastic wrap and refrigerate for at least 1 hour or up to 2 days.

Preheat the oven: When ready to bake, preheat the oven to 350°F (175°C). Line two baking sheets with parchment paper.

Shape the cookies: Use a medium cookie scoop to scoop out balls of dough. Roll them between your hands to form smooth balls, then place them on the baking sheets, leaving space between each cookie. Press extra M&Ms into the tops for garnish.

Bake the cookies: Bake in the preheated oven for 11 minutes, or until the edges are set but the centers remain soft.

Cool the cookies: Let the cookies cool on the baking sheet for 10 minutes before transferring them to a wire rack. While still warm, press a few extra chocolate chips or M&Ms on top for decoration.

Serve or store: Serve immediately or store in an airtight container at room temperature for up to 5 days. You can also freeze the dough or baked cookies for longer storage.