Position a rack in the lower third of the oven and preheat the oven to 350°F. Lightly grease the bottom and sides of a 9-inch springform pan.
Pulse the walnuts with 1 Tbs. of the sugar in a food processor until finely ground. Scrape the nuts into another bowl. Use a paper towel to wipe excess oil from the sides of the processor bowl, add the chocolate with 1 Tbs. sugar, and pulse until it forms crumbs ranging in size from coarse meal to 1/4-inch bits. Add the chocolate to the nuts, along with the orange zest, if using, and salt, and stir to combine. Set aside.
Using an electric mixer, beat the egg whites with the cream of tartar in a large clean, dry bowl at medium speed until the egg whites are creamy white and soft peaks are formed when the beaters are lifted. Gradually add the remaining 1/2 cup sugar, beating at medium-high speed until the egg whites are glossy and stiff but not dry.
Pour half of the chocolate mixture over the egg whites and fold in with a
large rubber spatula until nearly incorporated. Repeat with the
remaining chocolate mixture, folding just until evenly incorporated.
Scrape the batter into the prepared pan and spread it evenly. Bake for 25 to 30 minutes, until the torte is puffed and golden brown on top and springs back when gently pressed with your fingers; a toothpick inserted in the center should come out moist and possibly stained with melted chocolate, but not coated with raw batter. Set the pan on a rack to cool. Slide a thin knife or a small metal spatula around the sides of the torte to release it from the pan. Remove the pan sides and transfer the cake to a serving platter.
Serve slices with a dollop of whipped cream.
The torte can be kept at room temperature, covered or under a cake dome, for up to 3 days, or frozen, well wrapped, for up to 3 months; bring to room temperature before serving.