TO MAKE THE CAKE: In the top of a double boiler, over hot water, melt 6 oz. of the butter with the 6 oz of the unsweetened chocolate. When this is melted, remove it from the heat and transfer the mixture to a mixing bowl to cool to room temperature. While the chocolate and butter are cooling, lightly butter the bottoms of two 9-inch round cake pans and line the bottoms with parchment paper.
Preheat the oven to 350°F When the chocolate is cool, add the eggs, sugar, salt and vanilla and whisk together until well combined. Stir in the flour and pour the batter into the prepared baking pans and bake for 35 minutes or until a toothpick, when inserted in the middle comes out clean. Remove the cakes from the oven and cool them in the pans for 30 minutes. With a knife, loosen the edges of each of the cakes from the pans and turn them out onto a cake rack to cool completely.
TO MAKE THE ICING: While the cake is baking, prepare the icing. In a small saucepan, combine the sugar, instant coffee and heavy cream. Bring this to a boil, and boil gently for 6 minutes. Remove from the heat and add the unsweetened chocolate. After the chocolate has melted, add the butter and vanilla and cool to room temperature.
TO PUT THE CAKE TOGETHER: Set one of the cakes, top side up, on a plate and cover with a bit of the icing. Set the second layer upside down so the bottom is facing up and ice the top and sides. Pat the chopped nuts onto the sides of the cake, and center some in the middle of the top layer.
Refrigerate the remaining icing until firm. Transfer the icing to a piping bag fitted with a rosette tip and pipe rosettes around the edges. Store the cake in the refrigerator, but bring it back to room temperature before serving.