Mise en place. Line a 9" x 13" jelly roll pan with baking parchment paper. Preheat oven according to directions on cake box.
Mix red velvet cake mix, milk, melted butter, and eggs according to directions on box.
Pour into a greased 9" x 13" baking pan.
Bake according to directions on box.
Allow cake to cool.
Melt 3 C. of white chocolate baking bars for 30 seconds at 50% power. Stir. Repeat, stirring after each cycle. Be very careful not to overheat. Chocolate scorches easily! Spread over bottom of prepared jelly roll pan. Place in fridge for 15 minutes.
Trim edges off of cake. Crumble up half of the cake. Whip together cream cheese frosting and vanilla. Mix into cake.
Remove white chocolate from fridge. Spread cake/cream cheese mixture over top of chocolate, pressing down. Return to fridge.
Melt reminaing 3 C. of white chocolate baking bars for 30 seconds at 50% power. Stir. Repeat, stirring after each cycle. Be very careful not to overheat. Chocolate scorches easily! Remove pan from fridge. Carefully spread white chocolate over top. Sprinkle generously with red sprinkles.