3/4 cup (1.5 sticks) unsalted butter, softened to room temperature
1/2 cup granulated sugar
1/2 cup light brown sugar
1 egg, at room temperature
1.5 teaspoons vanilla
1 cup chocolate chips (I used a blend of white and semi-sweet chocolate chips)
1/2 cup sprinkles
How to do it:
In a large bowl, sift together flour, cake mix, and baking soda. Set aside. In a separate bowl, cream together the butter and sugars on medium speed. Mix in the egg and vanilla until creamy.
Add the flour mixture to the wet ingredients and mix until just combined. Do not OVERMIX the dough. Fold in the chocolate chips and sprinkles.
Cover and refrigerate dough for at least 1 hour, or up to 4 days. This step is mandatory. The dough is fairly sticky, so chilling the dough is required in order to avoid the cookie from spreading too much.
Preheat oven to 350 degrees.
Scoop rounded tablespoons of the cold dough onto an ungreased baking sheet. Shape your cookie dough balls to be "taller" than they are wide, as pictured above. Make sure to keep dough chilled when working in batches. Bake for 10-12 minutes until edges are slightly browned. (Mine took 10 minutes exactly). The centers will still appear very soft, but the cookies will set as they cool.
Allow the cookies to cool on the baking sheet for 4 minutes and transfer to a wire rack to cool completely.