Preheat oven to 350 degrees Fahrenheit. Lightly grease or line a 9x13 inch baking pan with parchment paper.

In a large mixing bowl, combine cake mix, egg, melted butter, and 0.5 cup heavy cream until just combined.

Spread batter evenly into the prepared baking pan. Bake for 20-25 minutes, or until a toothpick inserted into the center comes out clean.

Let the bars cool completely before frosting.

To make the frosting, beat together the cream cheese and butter in a large bowl until smooth and creamy.

Slowly beat in the powdered sugar, 0.5 cup at a time, until light and fluffy.

Add in the 3 tablespoons heavy cream and vanilla extract. Continue beating on high speed until a light, fluffy frosting of spreadable consistency is reached.

Separate the frosting into two bowls. Fold in purple food coloring into one bowl and blue food coloring into the other until desired colors are achieved.

Spread the frostings alternately over the cooled bars. Swirl with a spoon or flat spatula to create a marbled effect.

Top with sprinkles, if desired. Cut and serve.