Step 1: Melt the dark chocolate using a double boiler or microwave in short 20-second bursts, stirring until smooth. Be careful not to burn the chocolate.
Step 2: Stir in the butter while the chocolate is still warm until fully melted and blended, forming a silky, shiny mixture.
Step 3: Add brown sugar and whisk thoroughly into the chocolate-butter mixture. Beat in eggs one at a time until smooth. Stir in vanilla extract.
Step 4: Sift flour and salt over the batter. Gently fold with a spatula until just combined, taking care not to overmix.
Step 5: Butter three ramekins generously and dust with sugar. Tap out excess sugar to avoid clumps.
Step 6: Divide batter evenly among ramekins, filling about three-quarters full.
Step 7: Place ramekins in air fryer basket and cook at 180°C (356°F) for about 9 minutes until edges are set but centers remain soft and gooey.
Step 8: Let cakes rest 1–2 minutes. Run a knife around edges and invert onto plates carefully.
Step 9: Dust with icing sugar and serve immediately, optionally with vanilla ice cream or fresh berries.
Notes
Swap dark chocolate for milk or white chocolate for different flavors. Use gluten-free flour or coconut sugar for dietary preferences. Store leftovers in an airtight container in the fridge for up to 2 days. Reheat gently for best texture.