Finely grate white chocolate onto a plate using a vegetable peeler. Chill until needed.
Grease the base of two 8-inch round cake pans; line with parchment paper. Combine butter, sugar, flour, cocoa, baking powder, baking soda, salt and eggs in a large bowl. Using an electric mixer set on medium, beat until smooth. Continue beating until increased in volume, about 2 minutes longer. Add the hot water and beat to incorporate, about 1 minute longer. Divide batter between pans.
Bake cakes in a preheated 350-degree oven until a toothpick inserted in centers comes out clean, 20-25 minutes. Leave cakes in pans to cool for 10 minutes. Then turn onto a wire rack. Peel off lining paper; let cool.
Meanwhile, heat heavy cream just to a boil. Break chocolate into pieces; add to cream and stir until melted. Let cool; chill for 30 minutes, then whisk until thickened to a spreadable consistency.
Use a third of chocolate frosting to sandwich cakes. Spread the remainder over the whole cake. Sprinkle reserved grated white chocolate to garnish.