Prepare and bake your boxed cake according to package directions.
Prepare a square or rectangular baking dish with wax or parchment paper and set aside.
Whip together butter and cream cheese in a large mixing bowl. Add powdered sugar and mix until combined. Add vanilla and whipping cream or milk and whip until fluffy.
Add baked and slightly cooled cake to the frosting and mix until fully incorporated. Mixture will be thick and wet.
Spread the mixture in your prepared baking dish and cover with an additional piece of wax or parchment paper. Cover the dish with a lid or tinfoil and freeze at least 2 hours, preferably overnight.
When the cake mixture is frozen, cut into squares and pop them back into the freezer while you chop and melt your chocolate.
Drop each square, one by one, into the melted chocolate and flip to cover it completely in chocolate. Place on a piece of parchment or wax paper to cool and harden. If you will be adding something like sprinkles, make sure to do it while the chocolate is still wet.
If desired, decorate with drizzled melted chocolate in different colors.
Store in the fridge for up to a week, or in the freezer in a sealed container for up to 3 months.