Line an 8-inch square baking pan with parchment paper or foil, leaving enough overhang on two sides to lift out the chilled fudge later.

For current food-safety practice, place the measured dry cake mix in a shallow microwave-safe bowl. Microwave it in short intervals, stirring between intervals, until it reaches 165°F/74°C, then let it cool completely.

Place the white chocolate chips and sweetened condensed milk in a microwave-safe bowl. Microwave for 60 seconds, then stir thoroughly.

Continue heating in 15–30-second intervals, stirring after each interval, until the mixture is smooth. Avoid overheating the white chocolate.

Sift the cooled cake mix into the melted chocolate mixture and stir until fully incorporated. Fold in the sprinkles if using.

Spread the fudge evenly in the prepared pan. Sprinkle extra decorations over the top if desired, pressing them gently into the surface.

Refrigerate for at least 2 hours, or until firm. Lift the slab from the pan using the parchment overhang and cut it into 25–36 small squares.