Preheat oven to 300° F. Grease and flour a 10-inch springform pan.
Place egg whites and sugar in a mixer and whip into firm peaks.
Meanwhile, melt butter over medium heat. When melted, add chocolate, stirring occasionally until smooth. Quickly add vanilla extract, espresso, Kahlua, and chocolate butter mixture to the egg white mixture, gently folding until smooth. Pour into prepared cake pan and bake on center rack for 40 minutes.
Remove from oven. Wait 5 minutes and press sides down so the top of the cake is even. Cool and refrigerate for 4 hours or until set. Remove cake from pan and cut with a hot knife into 8 pieces. Serve with fresh berries and vanilla ice cream if desired.Makes 8 Source: