Step 1: Preheat your oven to 425°F (220°C).
Step 2: Grease four ramekins with butter and lightly dust with flour.
Step 3: In a double boiler or microwave-safe bowl, melt the chocolate and butter together until smooth.
Step 4: In a mixing bowl, whisk together the eggs and sugar until pale and slightly thickened.
Step 5: Slowly pour the melted chocolate mixture into the egg and sugar mixture, whisking constantly.
Step 6: Gently fold in the flour, vanilla extract, and optional coffee or espresso powder until just combined.
Step 7: Divide the batter among the ramekins and bake for 12-14 minutes.
Step 8: Let cool for 1 minute, then invert onto a plate and serve immediately.