In a large bowl, combine the flour, cake mix, baking soda and cornstarch. Whisk together and set aside.

In a microwave safe bowl, melt the 1.5 sticks of butter in 30 second increments. Once melted, allow to cool slightly. Whisk in both sugars and vanilla extract. Add the egg and egg yolk and whisk together until combined.

Pour in the dry ingredients and mix together using a rubber spatula or wooden spoon. Mix together until just combined. Fold in the chocolate chips and sprinkles. It may not look like either will stick because of the melted butter, but use those arms to work it in.

Cover tightly and refrigerate dough for at least 2 hours, or up to 3 days. This step cannot be skipped. If this step is skipped, the cookies will spread.

Preheat the oven to 350F degrees. Line two baking sheets with silicone baking mats (preferred) or parchment paper. Grab 1.5 Tablespoons of cookie dough and roll it into "taller" rather than round balls. Place 8 one a baking sheet at least 2 inches apart. Bake for 10-12 minutes. Mine took exactly 11 minutes. The cookies may look slightly under baked, but thats what you want. Allow to cool for a few minutes on the baking sheet before transferring to a wire rack to cool completely.

Cookies stay fresh up to 7 days in an airtight container.

Adapted from Best-Ever Chewy Chocolate Chip Cookies.