Cream the butter with 4oz of the sugar. Using an additional bowl, place a bowl over a pot of hot water and melt 5oz of chocolate. In a separate bowl, whisk egg whites until stiff, gradually pouring in 4oz of sugar. Stir the creamed butter, a tablespoon at a time, into the melted chocolate bowl. Add the ground hazelnuts and the remaining sugar into the melted chocolate. Remove chocolate from heat and hold in egg white mixture.
Place 6 wafers on a plate making a complete circle. Cover the wafers with the chocolate buttercream and cover with 6 additional wafers. Continue this pattern until all wafers and cream has been used. The top wafer will be covered in chocolate buttercream.
Melt the remaining chocolate over hot water (as down previously for the chocolate buttercream), add icing sugar and lukewarm water. Stir until smooth. Wait for chocolate icing to cool. Cover the top and sides of the cake with the chocolate icing. Chill the cake for several hours before serving.