Line an 8x8-inch pan with foil or parchment paper and spray with nonstick cooking spray (if using foil).

Place sweetened condensed milk and white chocolate chips in a medium saucepan. Place over medium-low heat and stir almost constantly until they melt together. This will takeseveral minutes; some of the white chocolate chips may not fully melt (see notes).

Remove from heat and stir in cake mix, vanilla, and salt. Carefully fold in sprinkles.

Pour fudge into prepared pan. Let sit at room temperature until cool, then chill to set.

Cut into squares. Store in an airtight container at room temperature for several days or in the refrigerator or freezer for several weeks.