Preheat the oven to 425 degrees. Butter a 9-inch springform pan. Line the bottom with a round of waxed or parchment and butter the paper.
In a 2 1/2 or 3-quart saucepan over medium-low heat, melt the chocolate and butter together, stirring until chocolate is about half melted. Remove the pan from the heat and stir until melted and the mixture is smooth. Set aside to cool for a few minutes.
Meanwhile, in a large mixing bowl, use a hand held or stationary electric mixer to begin beating the eggs at medium-low speed until frothy, then raise the speed to high and beat the eggs until they are light in color, thick, fluffy and about tripled in volume, about 5 minutes. Beat in the flour and espresso powder. Stir about 1/4 of the egg mixture into the chocolate to lighten it, then fold the chocolate mixture into the eggs in the mixing bowl.
Spoon the batter into the pan. Bake for 17 minutes. Cool the pan on a rack for about 10 minutes, then refrigerate until just firm enough to cut, about 30 minutes. (At this point, the cake will be firm and fudgy about 1/3 of the way to the center, and softly set like a mousse in the center.) The dessert can also be refrigerated for up to 5 days and served chilled and firm throughout, or frozen up to 1 month. Serve at cool room temperature or slightly chilled. Shortly before serving, whip the cream to firm peaks.