Preheat your oven to 350°F (175°C). Grease a 9-inch round cake pan with butter or line it with parchment paper.

In a medium saucepan over low heat, melt the chopped semisweet chocolate and cubed butter together, stirring until smooth and glossy. Remove from heat.

In a separate bowl, whisk the egg yolks with the sugar until thick and pale, about 3-4 minutes.

Fold the melted chocolate mixture into the egg yolk mixture until uniform in color.

In another bowl, beat the egg whites with cream of tartar until stiff peaks form.

Gently fold the beaten egg whites into the chocolate mixture in thirds.

Pour the batter into your prepared cake pan, smoothing the top.

Bake in the preheated oven for about 30-35 minutes, until the edges are set but the center is soft. Let cool completely before inverting onto a plate.

Optional: Dust the cooled torte with confectioners' sugar before serving.