Step 1Heat the oven to 450 degrees.
Step 2Combine the chocolate and butter in a double boiler or in a medium saucepan set in a larger saucepan half-filled with simmering water over medium-low heat. Melt the chocolate and butter together.
Step 3While the chocolate is melting, butter and flour four 4-ounce metal brioche molds or ceramic ramekins. Shake out excess flour, then put the molds on a baking sheet.
Step 4Whisk the eggs, egg yolks, sugar and flour with a stand or hand mixer until the mixture is pale and thick. When you lift the whisk, a ribbon of batter should fall on the surface of the remaining batter and stay there for a second.
Step 5By now, the chocolate and butter should be nearly melted. Remove the saucepan from the heat and whisk gently until smooth. While whisking gently, add the egg mixture in a slow, steady stream. Whisk just until incorporated, then divide the batter among the prepared molds.
Step 6Bake for 6 minutes. Pull them out and have a look. The edges should be set, but the center should be indented and a little wobbly. This is perfect. If you think the center is too fluid, return the tray to the oven for another minute. The good thing about this dish is that if you go too far, you wind up with a delicious brownie-like cake — it’s got so much butter it will never be dry — so mistakes on either end are fine!