Molten Mocha Cakes

Preheat oven to 400 degrees F. Butter two 6-ounce ramekins, then dust with sugar. Place butter and chocolate in a microwave-safe bowl. Microwave on high in 20-second increments, stirring after each, until melted. Let cool slightly. Meanwhile, in a small bowl, whisk together egg, egg yolk, sugar, espresso powder, and salt. Add chocolate mixture; whisk to combine. Add flour, and whisk just until combined (do not overmix). Pour batter into prepared ramekins. (Recipe can be made ahead up to this point.) Bake until a toothpick inserted 1/2 inch from edge of ramekins comes out clean, and a toothpick inserted in center comes out wet, 10 to 12 minutes (do not overbake). Cool 10 minutes on a wire rack. Run a knife around inside of ramekins to loosen. Invert cakes onto serving plates. Dust with sugar; serve immediately.