Preheat the oven to 200°C (400°F). Position a rack in the middle. Generously butter four 6-ounce (180 ml) ramekins and dust the insides with cocoa powder, tapping out excess. Place the prepared ramekins on a baking sheet.

Melt chocolate and butter

Roughly chop the Valrhona chocolate and cut the 150 g butter into pieces. Place both in a heatproof bowl set over a pan of barely simmering water (double boiler), making sure the bowl does not touch the water. Stir gently until completely melted and smooth. Remove from heat and let cool for 3–4 minutes.

Mix eggs and sugar

In a separate large bowl, whisk together the 4 whole eggs, 2 yolks and 150 g granulated sugar until the mixture is thick, pale, and ribbon-like when the whisk is lifted (about 2–3 minutes by hand, or 1–2 minutes with a handheld mixer). Stir in 1 tsp vanilla extract and 1 tsp fine sea salt.

Combine chocolate and eggs

Temper the egg mixture by pouring about one-third of the melted chocolate into the eggs while whisking constantly to avoid scrambling. Once combined, fold in the remaining chocolate mixture gently with a rubber spatula until uniform.

Add dry ingredients

Sift together 60 g all-purpose flour and 15 g Dutch-process cocoa powder. Fold the dry mixture into the chocolate-egg batter in two additions, folding just until no streaks remain. Do not overmix.

Portion batter

Divide the batter evenly among the four prepared ramekins (about 120–140 g batter each, depending on ramekin size). Smooth the tops with a spatula.

Bake

Bake on the middle rack for 10–12 minutes. The edges should be set and slightly puffed while the centers remain soft and slightly jiggly. Baking time can vary by oven and ramekin size; check at 10 minutes.

Finishing & Serving

Rest briefly

Remove the baking sheet from the oven and let the cakes rest 1 minute to settle. Run a small knife around the inside edge of each ramekin to loosen.

Invert and plate

Place a small serving plate over a ramekin and carefully invert to release the cake. If it resists, lift the ramekin and tap gently to help the cake release. Repeat for remaining cakes.

Finish and serve

Dust each warm cake lightly with confectioners' sugar if desired. Serve immediately with one scoop of quality vanilla ice cream placed beside or atop the warm cake so the ice cream melts slightly into the center.