Line an 8 or 9-inch square baking dish with parchment paper. Set aside.
In a large bowl (for a stand mixer), add white chocolate chips, yellow cake mix (the dry stuff) and the jar of marshmallow cream. Set aside.
In a large saucepan, melt butter with sugar, heavy cream and salt. Cook and stir over medium heat until mixture begins to boil.
Boil for 4 minutes, stirring continuously. Remove from heat and pour over white chocolate chips mixture in bowl.
Beat for about one minute using the whisk attachment, just until chocolate chips are melted. Fold in sprinkles and pour immediately into prepared baking dish.
Refrigerate for 4 hours (or overnight). Cut into bite size pieces and enjoy.
Notes
Best stored in refrigerator in air tight container for up to one week.