Step 1

In a bowl of a stand mixer, cream together the cold butter cubes and granulated sugar with the paddle attachment on low speed for 30 seconds. Increase speed to medium and continue creaming for 30 seconds, then increase to high speed and beat for another 30 seconds or until light and fluffy.

Step 2

Beat in the eggs, one at a time, followed by the vanilla extract. Add in the cornstarch, baking soda, salt, cake flour, and all-purpose flour and mix on low speed until just combined. Lastly, stir in the white chocolate chips and 1 cup of the rainbow jimmie sprinkles.

Step 3

Chill the dough for 30 minutes uncovered. While dough chills, preheat oven to 400° degrees F. Line 2 baking sheets with parchment paper or silicone liners.

Step 4

Using a digital kitchen scale for accuracy, measure out 6 ounces of cookie dough, shaping it roughly into a ball. Roll it loosely in the remaining sprinkles and place it on the baking sheet. Dough balls should be rougher textured and craggly looking, not smooth and compacted like a usual cookie dough ball. Place 3 more cookie dough balls 3" apart on the baking sheet, staggering them so they have plenty of space in between them.

Step 5

Bake one sheet of cookies at a time in the center rack of the oven for 10-13 minutes. Cookies will be slightly underdone in the centers but appear dull and set on the outside. Allow cookies to continue to cool on the baking sheets after baking for up to 30 minutes as cookies are quite fragile after being removed from the oven. While one tray of cookies bake, keep remaining cookie dough refrigerated for best results and to minimize spreading.