Ingredients :

• Dark chocolate (200 g) — This is what does all the work. Use at least 60% cocoa — anything lower and the cake will be too sweet and lack depth. Valrhona, Lindt Excellence, or an honest supermarket bar will do perfectly. This is not the time to buy the chocolate at 1 euro per kilo.
• Eggs — 4 whole + 2 yolks — This combination is what creates the signature texture. Whole eggs provide structure, while extra yolks add fat and that particular creaminess. Take them out of the fridge 30 minutes before — room temperature eggs incorporate much better into hot chocolate.
• Butter (150 g) — Unsalted butter, not semi-salted. It melts with the chocolate and creates a silky base. Get a decent one — this is also not the time to go low-end.
• Flour or cornstarch (50 g) — Just 50 g. This is intentional. Flour gives it a slight hold, cornstarch makes it even more melt-in-the-mouth. If you want the creamiest result possible, go with cornstarch.