Step 1 Beat the egg whites using a hand-held or stand mixer for 10 minutes on the lowest speed with 1 teaspoon sugar. Increase the speed to medium, and when the whites have reached soft peaks, slowly pour in the remaining sugar. Continue to beat to stiff peaks, but be careful not to overbeat.
Step 2 Melt the chocolate with the butter in a double boiler over medium heat until combined, about 10 to 12 minutes.
Step 3Whisk the egg yolks in a small bowl and fold them gently into the egg white mixture. Gently fold in the chocolate.
Step 4Line the bottom of an 8-inch springform pan with buttered parchment paper. Place the mixture in the pan. Bake the cake in a 350-degree oven for 25 to 35 minutes, until a toothpick comes out mostly clean -- there will be just a few sticky crumbs clinging to the toothpick.
Step 5Cool on a rack to just warm before unmolding. Serve slightly warm.