In a double boiler, or in the microwave, melt together the chocolate and butter. Allow to cool for a few minutes. In the meantime, separate the egg whites from the yolks. Add a small amount of the warm chocolate to the bowl with the yolks and stir to combine (this will temper the yolks). Then stir the yolks into the chocolate mixture.

Beat the egg whites until they become foamy. While the mixture is running, slowly add in the sugar. Beat until soft peaks form. Gently fold the egg whites into the chocolate mixture.

Generously butter the bottom and inside edges of a springform pan—or, as I like to call it, a cheesecake pan. Pour the batter into the prepared pan and bake at 275°F for 45 minutes. The cake should pull away from pan towards the end of baking. See? Allow to fully cool before you try to cut into the cake.