Line an 8×8 baking pan with aluminum foil or parchment and spray with nonstick spray. Set aside. Measure the white chocolate chips and the sprinkles in separate dishes so they are ready to add quickly.
Mix sifted cake mix and powdered sugar in a large bowl. Add milk and butter, without stirring, and microwave for 2 minutes.
Promptly mix ingredients until the butter is fully melted and incorporated. The batter will be very thick. Fold in white chocolate. Add the sprinkles last and stir gently so they don't leak too much color.
Scoop into prepared baking pan. Level it across the top. Chill the fudge in the refrigerator for at least 2 hours before cutting into small blocks.
Fudge will keep upwards of a week in the fridge, if it lasts that long.
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