For the crust: Grease the bottom of a 9 inch springform pan. Combine the wafer crumbs and melted butter; press onto the pan bottom firmly and evenly. Grease the sides of the pan, and attach. Preheat the oven to 350 degrees, setting a rack in the center.
For the batter: Combine the chocolate, butter and sugar in the top of a double boiler. Set over barely simmering water, and stir occasionally until the chocolate is melted and blended smoothly with the butter and sugar. Remove from the heat, and turn into a large mixing bowl; cool slightly; then add the egg yolks and beat just until incorporated. Stir in the grated orange rind. In the large blow of an electric mixer, beat the egg whites with the salt until soft peaks form. They should be firm but not dry. Stir about one-third of the egg whites into the chocolate mixture to lighten it. Fold in the remainder, lightly but thoroughly so that no white traces remain.
Turn the batter into the prepared pan and lightly tap two or three times on the counter to eliminate any large air bubbles. Bake for about 40 minutes, or until a thin crisp crust forms on the top and begins to crackle. the cake will quiver when lightly shaken. Remove from the oven, and place on a rack. Release the sides by running a sharp knife cleanly around the edges, leaving the pan sides attached. Before serving allow to cool for at least 1 hour, when it barely at room temperature; or cool completely, but serve within 6 to 8 hours to preserve the gossamer texture. Note: As cake cools, it will sink, forming a high irregular edge, which will be filled with whipped cream before serving. Although the cake is best served the same day it is prepared, it is also elicious buen denser if held overnight, or even several days. Do not refrigerate, but cover loosely with foil.
when ready to serve, prepare the topping. Pour cream into chilled mixing bowl; add the sugar and vanilla. Beat until the cream is thickened and spreadable (do not over beat or the cream will become buttery). Spread topping over the top of the cake to the edges. Garnish with curls of chocolate and candied orange peel.