In a large bowl, beat together unsalted butter, light brown sugar, and granulated sugar. Beat until ingredients are completely smooth and combined, and light and creamy. Add in the egg and mix until combined.
In a separate bowl, whisk together the baking soda, salt, instant vanilla pudding mix, Funfetti cake mix, and flour. Stir until combined.
Add the dry ingredients to wet. Beat till combined, but do not overmix. The cookie dough gets very thick. Gently fold sprinkles and white chocolate chips into the dough with a spatula.
Form round cookie dough balls that are 3 tablespoons (60 grams) in size. Cover and chill for at least 1 hour.
Preheat oven to 350°F. Line a baking sheet with parchment paper or a silicone baking mat.
Place no more than 6 cookie dough balls on a pan to give ample room for the dough to spread while baking. Bake for 9–13 minutes or until lightly browned at the edges and slightly underbaked in the center.
Remove from the oven and cool on the sheet pan for another 3–4 minutes. While they are on the sheet pan, you can add a few more white chocolate chips and sprinkles to the top, but this is optional.
Place cookies on a cooling rack until fully cooled. Cookies are best enjoyed the same day they’re made.
