In a medium bowl, whisk together the flour, cornstarch, baking soda and salt. Set aside.
In the bowl of a stand mixer fitted with a paddle attachment, beat on medium speed the butter, brown sugar and granulated sugar until light and fluffy, about 4 minutes. Scrape down the sides of the bowl when necessary. Add the egg, egg yolk and vanilla. Beat until combined, 20 to 30 seconds.
With the mixer on low, slowly add the flour mixture to the dough. Careful not to over beat. With a wooden spoon or rubber spatula, stir in the M&M’s and sprinkles by hand.
Cover cookie dough with plastic wrap and chill for at least 2 hours (or overnight).
When ready to bake, preheat oven to 350° F. Line a baking sheet with parchment paper and set aside.
Use a medium cookie scoop to scoop a heaping tablespoon of the chilled cookie dough. Shape it into a ball, and place onto the prepared baking sheet. Repeat placing each ball of cookie dough about 2-inches apart.
Bake for about 12 minutes or until the edges start to turn golden brown. Cool on the pan for 5 minutes and then transfer to a wire rack to cool completely. Store in an airtight container at room temperature.
Notes
Cornstarch: This is an optional ingredient that helps make cookies soft. If you don't have any on hand, then you can omit it from the recipe.Store cookies in an airtight container for up to 5 days.Freeze dough for up to 3 months. If the dough is rolled into balls, you can bake them from frozen for 14 to 15 minutes.