Preheat the oven to 350°F. Butter and flour a 9-inch springform pan.

In a double boiler over barely simmering water, melt the butter and chocolate and stir to blend. Let cool.

In a large bowl, beat the egg whites until foamy with a whisk or an electric mixer; add the salt and cream of tartar and beat until soft peaks form. Add the 1/4 cup granulated sugar and beat until stiff, glossy peaks form.

In a medium bowl, beat the egg yolks until thick and lemon-colored, then beat in the 1/2 cup granulated sugar and the coffee or spirits. Stir in the chocolate mixture until blended. Fold in one-third of the whites to lighten the mixture, then fold in the nuts, flour, and then the remaining whites. Pour the batter into the prepared pan and smooth the top.

Bake for 35 minutes, or until a cake tester inserted 1 1/2 inches from the center comes out clean (the center should still be soft). Let the cake cool completely in the pan on a wire rack before removing the pan sides.

Dust the cake with confectioners' sugar shaken through a sieve, and cut into wedges to serve.