Line a 9-inch square baking dish with parchment paper. Set aside.
In a large mixing bowl, add white chocolate chips, yellow cake mix (dry mix), and marshmallow cream. No need to stir. Set aside.
In a large saucepan, melt butter with sugar, heavy cream, and salt. Stir over medium heat until mixture begins to boil.
Once boiling, continue to stir constantly while a rolling boil continues for 4 full minutes.
After 4 minutes, remove from heat and pour over white chocolate chips mixture in bowl.
Beat for one minute on low until white chocolate is melted. Fold in 3/4 cup of sprinkles. Pour into prepared pan and sprinkle immediately with remaining 1/4 cup sprinkles.
Allow to set for 4 hours.
Notes
Store fudge at room temperature in airtight container. If desired, freeze in airtight freezer bag for up to 3 months.
Only cut pieces of fudge as needed to keep remaining block fresh.