1. Preheat oven to 450ºF. Butter 6 3/4-cup ramekins. Place chocolate and butter in a saucepan over lowest heat setting and cook, stirring constantly with a flexible spatula, until melted and smooth. Remove from heat, pour into a bowl and let cool slightly.
2. In a small bowl, mix flour, salt and espresso powder together with a fork. In a large bowl, using a wire whisk, beat eggs, yolks, cream and vanilla. Whisk in confectioners' sugar, just until combined. Fold chocolate mixture into egg mixture, then stir in flour mixture, just until combined (you shouldn't see any traces of flour, but try not to mix more than necessary).
3. Use a spoon to divide chocolate mixture among ramekins. Place ramekins on a baking sheet and bake for 10 minutes. Remove and let stand for 1 minute. Run a knife around inside edge of each ramekin and invert cakes onto plates. Serve hot, with a dusting of confectioners' sugar, if desired.