Preheat oven to 275 degrees. Line the bottom of a springform pan with parchment paper and butter the sides and parchment paper. It is important to grease the pan (sides and bottom) very well so that the cake doesn't stick.

In a double boiler, melt butter and chocolate together, once melted, remove from heat and cool.

Meanwhile, with an electric mixer beat egg whites until soft peaks form. Once soft peaks have formed, gradually add in the sugar and beat until you see glossy, stiff peaks.

Once chocolate is cooled, (it is very important that it is cooled as to not cook the eggs.) whisk in the egg yolks. Transfer chocolate mixture to a large bowl and whisk in ¼ of the egg whites into the chocolate before folding in the remaining egg whites. Be sure to fold them in well as to not have pockets of egg whites because this will cause the cake to react funnily while baking.

Pour batter into the greased pan, smooth the top with a spatula and bake for 45-50 minutes. Cool completely before removing sides of the pan, and serve.