Serve in plain wedges, with whipped cream and sauce(s), or frost the cake with whipped cream and piped designs, then serve in a pool of sauce.
Special equipment (if you plan to frost the cake with whipped cream)
9-inch corrugated cake circle
Pastry bag fitted with Ateco #7 closed star tip
Preheat oven to 425. Line 8x2-inch round cake pan with parchment or waxed paper.
Melt the chocolate and butter in a small bowl placed in a barely simmering water bath over low heat, stirring occasionally until smooth. Or, microwave on medium (50%) for about 3 minutes and 50 seconds. Stir until completely melted and smooth. Whisk in egg yolks and flour. Set aside.
Beat egg whites with cream of tartar in a clean, dry mixing bowl at medium speed until soft peaks form. Gradually sprinkle in 1 Tbsp sugar, beating at high speed, until stiff but not dry. Fold about one-quarter of egg whites into the chocolate mixture to lighten it. Quickly fold in remaining whites. Turn mixture into prepared pan. Bake for exactly 15 minutes.
Remove from oven and cool in pan. Cake will rise somewhat, especially around edges, but will still seem undercooked. At it cools, it will sink in the center. This is correct! Let the dessert cool completely in the pan on a rack. Cover and chill for several hours in the refrigerator.
Run a knife or small spatula around the edge to release it from pan. Warm the bottom of the pan on a stove burner for just a few seconds; invert it onto a plate to unmold. Peel the parchment liner from bottom and invert again onto serving plate or corrugated cake circle (if you plan to frost it with whipped cream). The cake circle will be 1/2 inch bigger than the dessert all around. The dessert will be right-side-up and higher around the edges than in the middle. It may be made to this point up to 4 days in advance. Wrap well and refrigerate until needed, or freeze for up to 3 months.
To serve simply: Remove from refrigerator at least 1 hour in advance. Cut in wedges and pass lightly sweetened whipped cream and sauces separately.
To serve more formally: Whip cream with remaining 3 Tbsp of sugar and mask cake thickly with about 3/4 of it. Scrape the remaining whipped cream into pastry bag and pipe large rosettes or swirls around the top edge of the cake.
From the Dinner Co-op