Preheat oven to 350 degrees. Line a baking sheet with parchment.
Add the butter, brown sugar, and granulated sugar to a mixer and beat until well combined.
Beat in the egg and vanilla.
Add the flour, cake mix, baking soda, and salt to the mixer and mix until just combined.
Stir in the milk and white chocolate chips and sprinkles until combined.
Use a medium cookie scoop to drop spoonfuls of dough on the prepared baking sheet.
Bake for 10 minutes or until edges are set but centers are still soft and slightly underdone.
Cool on the baking sheet for at least 5 minutes before serving.
Video
Notes
This dough is not as stiff as a lot of cookie doughs. It's easy to mix this one by hand using a wooden spoon. A mixer will also work fine. The cookies should look a bit under baked when you remove them from the oven. They will continue firming up as they cool on the cookie sheet.