Prehat the oven to 180ºC . Butter and flour a 25 cm springform pan.
Heat the chocolates and butter in a double boiler over simmering water, stirring frequently, until melted. Remove from the heat and cool to room temperature, stirring occasionally. about 25 minutes. Beat the eggs in a large mixer bowl until frothy. Gradually add the sugar and beat until mixture becomes thick and lemon-colored and ribbon forms when the beaters are lifted, about 10 minutes. Add the vanilla. Gradually add the chocolate mixture and beat well. Fold in the flour and pour the batter into the prepared pan. Bake 45 to 50 minutes, until the top of the cake begins to crack. Cool completely in the pan on a wire rack, then wrap and refrigerate overnight. Let stand at room temperature for 1 hour before serving. Remove the sides of the pan and place on a serving plate. Sprinkle the top with confectioners' sugar. For best results, cut the cake with a knife dipped in warm water, wiping off the knife between cuts.
Yield: 16 Servings
NOTE: This very densed chocolate cake is named after the famous California fault becaus it forms big cracks as it bakes and cools. Adjust the oven rack to the lower third of the oven.