Melt chocolate and butter in top of a double boiler over hot, not boiling, water, stirring until smooth. Remove from heat; cool. Combine eggs and sugar in a large bowl. Beat at medium speed with an electric mixer until well blended. Gradually add cooled chocolate, beating at low speed until blended.
Pour batter into a buttered, wax paper-lined, 9-inch deep round cake pan. Place cake pan in a large shallow pan. Add hot water to shallow pan to depth of 1 inch.
Bake at 350° for 55 minutes. Turn oven off; let stand in oven 50 more minutes or until set. Remove cake pan from water bath, and cool cake completely in pan on a wire rack. Cover and chill 2 hours. Loosen edges of cake with a spatula, and invert cake onto a serving plate. Cut into wedges, and pipe or spoon Espresso Cream on top. Top with mixed berries and mint sprigs.