Break the dark chocolate into small pieces. Place the pieces into your glass bowl. Set the bowl over a simmering pot. Do not let the bowl touch the water. Stir the chocolate until it is very smooth. Add the small knob of butter now. Stir it until the butter melts completely. Take the bowl off the heat. Let the chocolate cool for five minutes. It should be warm but not hot.

You need to be very careful here. Crack the eggs one by one. Put the whites in a large bowl. Put the yolks in a small cup. Do not get any yolk in whites. The whites must stay very clean. This helps them grow big and fluffy.

Add the yolks to the melted chocolate. Do this while the chocolate is still soft. Stir them in very quickly. The mixture will look thick and shiny. Add the vanilla extract to this mix. Add the pinch of sea salt now. Stir it all together until it is smooth.

Use your electric mixer for this part. Turn it on to a medium speed. Beat the whites until they look bubbly. Add the sugar very slowly now. Turn the mixer to a high speed. Keep beating until you see stiff peaks. The whites should look like white snow. They should stay in the bowl when flipped.

Pour the cold cream into another bowl. Whisk it by hand or with mixers. It should become thick like soft clouds. Do not whisk it for too long. It should not turn into butter. Set the whipped cream aside for a moment.

Add one spoonful of whites to chocolate. Stir it in to loosen the mix. Now add the rest of the whites. Use a very gentle folding motion. Move the spatula in a big circle. Do not stir it like a soup. You want to keep the air inside. Stop when you see no white streaks.

Add the whipped cream to the chocolate. Use the same gentle folding motion again. Be very careful not to pop bubbles. The mix should look light and brown. It will feel very airy and soft.

Spoon the mousse into your glass jars. You can also use small bowls. Smooth the tops with a small spoon. Put them in the fridge right away. Let them chill for two hours. This helps the mousse set firmly.