Place the cake on a cake board or the base of the cake pan. For easy and smooth frosting, crumb coat the cake, then freeze for 30 minutes.
Place the chilled crumb-coated cake on a turntable. Using an offset spatula, apply a few scoops of the frosting on top, then down to the side of the cake. Smooth the top of the cake with a spatula.
When the sides are covered, hold the cake scraper about 90° angle parallel to the cake or as tight and close to the cake while you rotate the turntable. Do this until the cake is smooth.
Freeze for 30 minutes or until the chocolate mirror glaze is ready to pour.
For the gelatin. Sprinkle the powdered gelatin in a small soy dish with water. Let it stand to bloom for about 5 minutes. Microwave until melted and let it cool.
For dark chocolate and milk chocolate. Make the chocolate mirror glaze. Heat the heavy cream and pour over the chocolate in a bowl. Let it stand for 3 to 5 minutes until the chocolate softened. Gently stir in the melted gelatin until blended. Cover the surface of the chocolate with cling wrap to prevent skin from forming. Let it cool until the temperature reaches between 90°F to 87°F.
Straining the chocolate mirror glaze. Using a fine-mesh strainer over a liquid measuring cup. To avoid bubbles and lumps, pour slowly and strain the chocolate mirror glaze.
Glazing the cake. Place the chilled cake over a small bowl or a wide mouth glass on a baking tray to catch excess glaze. Pour the chocolate mirror glaze on top of the cake up to the cake edges, then pour the white chocolate mirror glaze in a zigzag motion to make a marble effect.
Removing the drips. Let it drip for 5 minutes before removing the excess drips with a hot spatula or knife.
Moving the cake. You will need 3 spatulas to move this cake. Place the 2 big spatulas under the cake close to the edge. Carefully lift the cake with 2 big spatulas and place it on the cake stand or cake plate. Slightly lift one spatula, then slide in the small spatula under the cake board to release the bigger spatula. Repeat on the other spatula.
To clean and fix the glaze. Dip the spatula in hot water, then wipe it off with a paper towel to clean the edges of the cake.
Slicing the cake. For clean, smooth and mess-free slices, use a hot knife to slice the cake.