Preheat oven to 325 degrees.
Butter a 5-cup soufflé mold. Coat inside with teaspoon sugar. Combine espresso with hot water, stir until dissolved.
Melt chocolate on top of double boiler. In a mixing bowl, combine melted chocolate with cream and coffee.
Beat egg yolks and flour. Stir into the chocolate mixture.
Beat egg whites with cream of tartar to a soft peak. Sprinkle in sugar, and beat until stiff peak. Stir a 1/4 of the whites into chocolate mixture, then fold chocolate mixture into remaining egg whites, a little at a time. Pour this into the prepared mold.
Bake for 35 45 minutes. Serve hot with lightly whipped cream.