In a large bowl, whisk together the flour, baking soda, cornstarch and salt. Set aside.
In a medium sized bowl, whisk together the melted butter, brown sugar, and granulated sugar. When sugar clumps disappear, whisk in the egg, egg yolk, and vanilla.
Pour wet ingredients into dry ingredients and mix together with a large spoon or rubber spatula.
At this point, stop to reserve some of the dough for the inside of the cookies. You will reserve 1/4 cup + 2 Tablespoons of the dough. To this dough, add the 1/4 cup of sprinkles in your color of choice (for the inside of the cookies-- the reveal color). Chill this dough at least 1 hour.
Fold the chocolate chips and the mixed sprinkles into the remaining cookie dough. Cover dough and chill in the refrigerator for at least 2 hours, or up to 3 days.
After the inside dough has chilled for an hour, roll it into balls 1 teaspoon in size. You will have about 18 mini dough balls. Freeze these dough balls for the remaining hour.
When you are ready to bake the cookies, preheat the oven to 350ºF degrees. Line two large baking sheets with parchment paper or silicone baking mats and set aside.
Roll the chilled dough into balls-- 1 ounce (approximately 1 Tablespoon) of dough each-- and place 8 balls of dough per cookie sheet. Keep dough you are not rolling or baking in the refrigerator.
Using your thumb, press a well into the center of each dough ball.
Place 1 frozen dough ball into the center of 4 of the dough balls. Place another dough ball onto the top of the frozen dough ball so you make a cookie "sandwich."
Using your fingers, mold the cookie dough sandwich into a large but uniform ball of dough. If you'd like, lightly roll the tops of each dough ball into the mixed sprinkles for added flair to the top of each cookie. You should have 4 large cookie dough balls on your tray before baking.
Bake the cookies for 14-15 minutes or until edges just start to turn brown. Remove from oven and allow cookies to cool on the baking sheet for 10 minutes before moving to a wire rack to cool completely. Store cookies at room temperature, in an airtight container, up to 7 days. Baked cookies may be frozen up to 2 months. You may also freeze rolled cookie dough up to 2 months. Bake frozen for 15-16 minutes.
*Room temperature egg and yolk are preferred for even distribution into batter. As a rule of thumb, when a recipe calls for room temperature or melted butter, it's generally a good idea to use room temperature eggs as well. If you don't have time to let eggs come to room temperature, place them in a bowl of warm water for 5-10 minutes before using.