One 10-ounce jar bittersweet chocolate sauce (1 1/4 cups)
One 9-ounce package chocolate wafers
Shaved bittersweet chocolate, for garnish
In a large chilled stainless steel or glass bowl, using a handheld electric mixer, beat the heavy cream with the chocolate sauce at medium speed until firm peaks form. Spread about 1/2 cup of the whipped chocolate cream on a long rectangular platter, to form a 3-by-10-inch rectangle.&
Using a small offset spatula, spread 1 tablespoon of the remaining chocolate cream on 35 chocolate wafers and arrange them in 5 stacks. Top each stack with a chocolate wafer (you will have 6 or 7 wafers left over). Arrange the wafer stacks on their sides as close together as possible on the chocolate cream on the platter. (The wafer stacks will lie lengthwise on the platter.) Spread all but about 1/2 cup of the remaining chocolate cream all over the cake, fixing any wafers that tilt or slide.
Press a long sheet of plastic wrap over the cake, flattening the top and sides gently. Refrigerate for at least 8 hours or for up to 2 days. Refrigerate the remaining chocolate cream.
Discard the plastic wrap and frost the cake with the remaining chocolate cream, smoothing the top and sides. Garnish with the chocolate shavings. Cut into slices, wiping the knife after each cut.
The finished cake can be refrigerated in an airtight container for up to 2 days.