Step 1In a small saucepan over low heat, warm the milk until it is just hot to the touch, but do not let the milk come to a boil. Keep warm.
Step 2In a small bowl, combine the ground espresso and flour, tossing with a fork to mix well. Set aside.
Step 3In a mixing bowl, add the sugar to the egg yolks and beat until the mixture is light-colored and thickened. Continue beating as you slowly pour in the warm milk. Add 1 teaspoon vanilla, the coffee and the mixture of ground espresso and flour, beating constantly.
Step 4When everything is well-combined, transfer the mixture to a heavy saucepan over low heat. Cook, stirring constantly, until the mixture thickens to a creamy consistency. Note that it's important to watch the mixture and stir constantly because if it comes to a boil, it will curdle as the eggs start to cook. (If that starts to happen, immediately pull the pan off the heat and cool it by setting the base of the pan in cold water. Strain the mixture to get rid of any curdled/cooked bits and continue with the remainder.) In the end, the mixture will thicken to a custard and coat the back of a spoon. Remove from the heat and set aside to cool.
Step 5Beat 1 cup of the whipping cream in a chilled bowl to soft peaks, then fold the whipped cream into the coffee mixture. Rinse the beaters and beat the egg whites to stiff peaks; fold into the coffee cream.
Step 6Pour into a glass bowl and freeze about 1 hour, then stir the mixture, which should be half-frozen. Divide into espresso cups and return to the freezer until almost frozen, an additional 2 to 3 hours.
Step 7Whip the remaining cream together with the powdered sugar and remaining vanilla to soft peaks. Spoon a small dollop of whipped cream onto each serving and sprinkle lightly with bittersweet chocolate. Serve immediately.