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

Preheat your oven to 350°F. In the top of a double boiler or a metal bowl set over barely simmering water, melt the semisweet chocolate and butter together. Stir until the mixture is smooth. Remove from the heat and let it 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 until well combined.

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 after each addition until sugar dissolves. Continue beating until stiff, glossy peaks form.

Fold one-fourth of the whipped egg whites gently into the chocolate mixture to lighten it. Then carefully fold in the remaining egg whites until fully incorporated without deflating the mixture.

Transfer the batter to a greased 9-inch springform pan. Bake in the preheated oven 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. Loosen the sides of the pan with a knife, then remove the rim of the springform pan. If desired, dust the top with confectioners' sugar before serving.