Preheat oven to 350, with oven rack on lower third shelf; butter bottom of 9×3-in springform pan, cover it with parchment, then butter-and-flour the parchment circle.
3/4 c. superfine sugar, divided
1 1/2 c. walnut halves or pieces, lightly toasted & cooled
5 oz. dark chocolate, chopped (I used Ghirardelli 60% chocolate chips)
8 large eggs, separated
1 t. cream of tartar
In bowl of food processor, pulse walnuts with 2 tablespoons of the sugar until very fine. (Stop before nuts become oily.) Empty into a medium bowl, then process chocolate until very fine (but not beginning to melt). Add to nuts and combine.
In bowl of stand mixer fitted with a whisk attachment, beat egg whites until foamy. Add cream of tartar and continue beating, gradually adding 2 tablespoons of sugar until stiff peaks form. Empty into large bowl and set aside.
Add egg yolks and remaining 1/2 of sugar to empty mixer bowl and beat on high speed for 5 minutes until very thick. On slow speed, add chocolate/walnut mixture and beat just until blended.
With rubber spatula, gently fold in the beaten egg whites in three parts until no streaks remain. Scrape mixture into prepared pan and bake 35 – 45 minutes until cake springs back when pressed very lightly in the center.
Put pan on wire rack and let cool 5 minutes, then run a knife around the sides and remove the side of the pan. Cool completely – then invert onto cooling rack, remove parchment, and invert onto serving platter.
To serve: Cake can be served simply (with a light dusting of powdered sugar or a drizzle of chocolate glaze, or a dollop of whipped cream and garnish of fresh fruit). Or it can be dressed up, however you like. For our meeting, I brushed the cake with warmed apricot preserves, then covered it with about 1 1/2 cups of heavy cream whipped with a little vanilla and powdered sugar, and sprinkled chocolate shavings on the top and sides.