Pour cake mix and sweetened condensed milk into a large bowl and stir to combine.
Add white chocolate chips, then microwave for 1 minute and stir. Place back in microwave for 30 seconds and stir again. Place back in for 30 more seconds and gently stir, making sure chips are fully melted.
Fold in sprinkles, being careful not to stir too much. (you don't want to sprinkles to melt and for the color to bleed)
Scoop mixture into an aluminum foil-lined 8”x 8” baking dish (the foil will make it easier to pull out later).
Top with additional sprinkles.
Refrigerate for at least 2 hours, or until set. Cut into 1” bars and serve.