1. Cut the 9- x 13-inch cake into one-inch strips, then cubes. In a large bowl, combine cake cubes with one and a half to two cups chocolate cream. Mix until the cake and cream are thoroughly combined. You want to mix carefully so it doesn’t get too mushy.
2. Meanwhile, flip the flat cake onto another baking sheet and peel off the parchment. Spread two-thirds of a cup of the chocolate cream in a thin layer over the cake.
3. Slice the flat cake in half horizontally. Add the filling in an even layer over one half. Turn the other layer over, on top of the filling.
4. Prepare the glaze. Whisk together confectioners’ sugar, oil, and grape juice, and slowly add in water. Add more sugar or water if needed to reach glaze consistency. Spread glaze over the entire top of the cake.
5. Slice the cake in half into two long log shapes. Ice the sides of the cake with remaining chocolate cream (it’s easier to get it perfectly smooth if you freeze the cake and ice the sides of the cake a second time then). Pipe with whipped cream/topping along the edges for garnish.