Preheat oven to 175°C / 350°F (for fan oven - 160°C / 320°F). Place 4 ramekins in a tray. If you wish to unmould the cake, butter and dust the ramekins with cocoa powder. If you are serving the cake in the ramekins directly, there's no need to do anything to the ramekins.
Double Boiler MethodPlace butter and chocolate in a heatproof bowl. Heat a small pot with 1-inch of water to simmering and place the bowl on top. The bottom of the bowl should not be touching the water. Whisk frequently, until butter and chocolate has melted. Remove from heat.Microwave MethodPlace butter and chocolate in a microwave bowl. Microwave for 20 seconds and whisk the mixture. Repeat until butter and chocolate has melted.
Whisk in caster sugar - it will be a little grainy but it's ok. Let mixture cool to room temperature before whisking in the egg yolks until fully incorporated. Set aside.
Place egg whites in a dry and grease-free bowl. Using an electric mixer, beat on medium speed until soft peaks. Beat in sugar in 2 batches, then beat on medium-high speed until stiff peaks.
Fold the meringue in 3 batches into the Chocolate Batter gently.
Divide the batter among the ramekins. Bake for 15 minutes until the cakes are puffed, tops are cracked and the cakes are no longer jiggly. Do note not to overbake.
Let the cakes cool and serve warm or at room temperature. Serve with a dust of cocoa powder, or for more indulgence, chocolate sauce, whipped cream and/or ice cream!
Store leftovers in a container in the refrigerator. Let the cake come to room temperature (or warm it in the microwave) before serving.
The key to make this delicious is to use best-quality dark chocolate you can find. I used Valrhona chocolate for this recipe.