Preheat the oven to 200°C (400°F). Grease 4 small ramekins with butter and lightly dust with flour.

Melt chocolate chips and butter in a heatproof bowl over a pot of simmering water, stirring until smooth. Remove from heat.

Whisk in powdered sugar, then add eggs one at a time, mixing thoroughly. Stir in vanilla extract and flour until just combined.

Divide the batter evenly among the prepared ramekins. Place the ramekins on a baking sheet and bake for 8-10 minutes, or until the edges are set but the center is still soft.

Let the cakes cool for 1 minute, then gently invert them onto serving plates.

Dust with powdered sugar and garnish with fresh berries or ice cream if desired. Serve immediately.