Melt the chocolate and butter on a water bath or in a microwave. Mix well.
Add sugar and mix well.
Separate yolks from egg whites.
Add yolks into the melted chocolate and mix.
Add salt to the egg whites, and beat into a thick foam.
Using a spatula carefully fold in whipped egg whites into chocolate mass.
Preheat oven to 380 degrees F. Generously butter a baking pan and pour the batter into the cake pan.
Bake in in the preheated oven for about 25 minutes. The exact time of baking depends on the oven, size and quality of the baking pan. Once the the cake is moist and not liquid inside (it's not a lava cake) - it's ready. Please, don't overcook!
Let the cake cool then carefully remove it from the pan. Dust it with powdered sugar and serve.