Preheat the oven to 350F (180C). Line cookie trays with parchment paper or baking mats.
In a large bowl, beat together the butter, brown sugar and granulated sugar until creamed. Do not overbeat - this can lead to the cookies spreading too thin. About 1-2 minutes is more than enough.
Then mix in the egg and vanilla extract until you no longer see pieces of egg.
Add in the flour, cornstarch, baking powder, baking soda and salt. Do a few stirs by hand first, then beat the mixture together starting with the mixer on a low speed until combined.
Add in the chocolate chips, white chocolate chips and sprinkles. Stir into the batter using a rubber spatula or wooden spoon.
Spoon the dough into balls with about 1.5 to 2 tablespoons of dough each. Place cookie dough balls about 2 inches (5 cm) apart on the lined cookie sheets.
Bake 1 cookie sheet at a time in the middle of the oven for about 10-12 minutes, or until the tops look just set.
Cool the cookies on the cookie sheet for at least 10 minutes before transferring to a wire rack to continue cooling. Optionally, add a few extra chocolate chips on top of each cookie. Be sure to add the chocolate chips when the cookies are fresh out of the oven and still warm.