1. Preheat oven to 180c/360f. Lightly grease an 8" springform pan; cut a piece of parchment to fit, grease it, and lay it in the bottom of the pan. Set aside.
2. Melt chocolate with butter in a medium bowl set over a saucepan of barely simmering water, stirring until smooth. remove from heat and let cool.
3. Separate egg yolks and egg whites. Add the egg yolks to the chocolate mixture and stir until incorporated and smooth.
4. beat egg whites with pinch of salt until soft peaks form. Gradually add sugar and continue beating until stiff peaks form. Whisk 1/4 of the egg whites into the chocolate mixture, then gently fold in remaining egg whites.
5. Pour batter into the prepared pan. Bake for 20 minutes. Let cool completely and refrigerate for at least 4 hours. Before serving dust with cocoa powder or powdered sugar.