Ho Ho Cake

Prepare the cake batter as directed on the box. Bake chocolate cake in a sheet pan (12 x 17 x 1 inch pan) according to package instructions. Cool cake completely.
Prepare "That's the Best Frosting I've Ever Had" recipe. See MissyDew's TastyKitchen recipe box for the recipe. You could get by with half of the frosting recipe if you want a thinner layer of frosting. A thick layer of frosting makes a very rich cake. Very rich isn't a bad thing, but it's your call. Spread the frosting on the cooled chocolate cake.
For the chocolate topping:
(You will want to spread the chocolate topping on the cake immediately once it's prepared to avoid a grainy texture-it sets up fast. So make sure the cake is cooled and frosted before you start to make the topping)
Put the chocolate chips and chopped chocolate into a heatproof bowl.
Heat 3/4 cup heavy cream in a saucepan over medium heat. Once it's hot, pour hot cream over chocolate chips and chopped chocolate. Let it sit a minute or so, then stir gently to melt.
Cream the butter and 1/2 cup of the powdered sugar in a large bowl. Stir the melted chocolate/cream into this. Once combined, add the remaining 1 cup powdered sugar. Add an additional 1/4 cup heavy whipping cream for a slightly thinner frosting. Spread on cake immediately!
Allow to set up before cutting and serving. Can be stored at room temperature.
Recipe adapted from several sources.