Start by prepping your ramekins. Grease four 8-ounce ramekins with butter and then dust the insides with a bit of flour, tapping out any excess. This will help the cakes release smoothly after baking.
Next, set a mixing bowl over a medium saucepan filled with about one cup of water (make sure the bowl doesn’t touch the water). Add the semisweet chocolate and the sliced butter to the bowl. Turn the heat to low and stir continuously using a spatula until the mixture is melted and smooth. Once fully melted, carefully remove it from the heat and set aside while you work on the egg mixture.
In a separate bowl, combine the eggs, egg yolks, sugar, and salt. Using an electric mixer, beat the mixture on high speed for about five minutes. You want it to become thick, pale, and fluffy. This step is crucial for creating a light texture in the final product.
Now, gently pour the warm chocolate mixture into the egg mixture. Add the vanilla extract and fold the two mixtures together until they are mostly combined. Be careful not to overmix; you want to keep as much air in the batter as possible.
Next, sift in the flour and fold it gently until fully incorporated. The batter should be smooth and glossy.
Divide the batter evenly between the prepared ramekins. Be sure not to overfill them; about three-quarters full is perfect.
Preheat your oven to 450 degrees Fahrenheit (about 230 degrees Celsius). Once heated, carefully place the ramekins in the oven and bake for 10 to 12 minutes. You’ll know they are done when the outside appears firm to the touch, but the centers still have a slight jiggle.
After baking, remove the ramekins from the oven and let them cool for about one minute. To release the cakes, run a small knife around the edges. Then, invert each ramekin onto a plate to reveal the beautiful, molten center.
Dust with powdered sugar on top and serve immediately. Watching the chocolate lava flow as you cut into them is the best part!