01 - Preheat oven to 425°F. Grease four 6-oz ramekins with butter and dust lightly with flour or cocoa powder, tapping out excess.
02 - Combine dark chocolate and butter in a heatproof bowl over simmering water or microwave in short bursts; stir until smooth. Let cool slightly.
03 - In a medium bowl, whisk eggs, yolks, and sugar until pale and thick, approximately 2 minutes.
04 - Gently fold melted chocolate mixture into the egg mixture until just incorporated.
05 - Sift flour and salt into the batter, folding carefully until smooth without overmixing.
06 - Divide batter evenly among prepared ramekins and place on a baking tray.
07 - Bake for 11 to 12 minutes until edges are firm but centers remain soft. Avoid overbaking to preserve molten center.
08 - Simmer raspberries, sugar, and lemon juice in a saucepan over medium heat for 3–4 minutes until berries break down. For thicker coulis, stir in cornstarch dissolved in 1 tsp water and continue cooking for 1 minute.
09 - Remove from heat and strain mixture through a fine sieve to eliminate seeds. Allow to cool.
10 - Let cakes rest 1 minute after baking. Run a knife around edges, invert onto plates, dust with icing sugar, and serve immediately with raspberry coulis and optional fresh raspberries.