Preheat oven to 350.

Line the bottom of the spring-form cake pan with parchment paper.

Melt the chocolate in the microwave on low, then let the butter melt in the warm chocolate.

Beat the 2 whole eggs and 4 egg yolks with 1/3 cup of the sugar, then gently add the chocolate mixture and orange zest.

In another bowl, whisk the 4 egg whites until foamy, then gradually add the remaining sugar and whisk until the whites hold their shape but are not too stiff. Fold in the egg whites to the chocolate mixture.

Pour into the prepared pan and bake for 35 minutes. Cool the cake in its pan on a wire rack; the middle will sink as it cools.

When you are ready to eat, place still pan-bound cake on a cake stand or plate for serving and carefully remove the cake from its pan. Fill the center with freshly whipped cream, or serve plain with ice cream.