Serving suggestion: Sweetened or flavored whipped cream.
Heat the oven to 375 degrees F. Line with parchment, grease, and flour an 8-inch/22 cm springform pan. Melt the chocolate gently over a water bath, and then beat in the butter a piece at a time until smooth. In a separate bowl, beat the yolks with 1/2-cup sugar until thick, pale, and ribbony. In yet another bowl, beat the whites to soft peaks. Scatter over the remaining 1/2-cup sugar and beat to a stiff meringue. Slowly whisk the chocolate mixture into the yolk mixture. Stir in a spoonful of beaten egg whites, then pour the chocolate mixture over the egg whites and gently fold together with a spatula. Pour the batter into the prepared pan and bake 50 minutes. Remove from the oven. Run a knife around the outside edge, then let sit until cool. It will sink down and the top will crack, appealingly. Serve with a drift of slightly sweetened whipped cream, flavored with vanilla, rum, or orange flower water.