Place the egg whites in a large bowl and let them stand at room temperature for 30 minutes.

Preheat your oven to 350°F. Melt the semisweet chocolate and butter together in a double boiler or a metal bowl set over barely simmering water until smooth. Remove from heat and let cool slightly.

In another large bowl, beat the egg yolks until thick and lemon-colored, then beat in the slightly cooled chocolate and butter mixture.

Using clean beaters, beat the egg whites with cream of tartar on medium speed until foamy. Gradually add sugar one tablespoon at a time, beating on high speed until dissolved and stiff glossy peaks form.

Fold one-fourth of the whipped egg whites into the chocolate mixture to lighten it, then fold in the remaining egg whites until fully incorporated.

Transfer the batter to a greased 9-inch springform pan. Bake for 40-45 minutes, or until a toothpick inserted in the center comes out with moist crumbs.

Let the torte cool completely on a wire rack, then loosen the sides with a knife and remove the rim of the springform pan. Dust with confectioners' sugar if desired before serving.