Position rack in center of oven and preheat to 350 degrees. Lightly butter a 9" springform pan. Dust pan with flour and tap out excess. In top of double boiler over hot, not simmering water, melt together espresso and chocolate, stirring occasionally. Remove pan from heat and cool until tepid. In large bowl, using hand-held electric mixer set at medium high speed, cream sugar and butter together until light and fluffy. One at a time, add egg yolks to mixture, beating well after each addition. Beat in flour. In large grease-free bowl, using hand-held mixer at medium high speed, beat egg whites until they form stiff, shiny peaks. Fold 1/4 of the whites into chocolate mixture to lighten. Then fold in remaining whites. Fold in butter and flour mixture. Scrape batter into prepared pan and bake 60 to 70 minutes or until top is crusty and cracked and the middle is still slightly moist. Remove cake to wire rack to cool completely. Remove sides of springform pan and transfer cake to serving plate. If desired, place doily on top of cake and sprinkle with confectioners sugar. Remove doily to form pattern.