Preheat the oven to medium heat (180 degrees). Butter a baking tray (30×400 mm) and dust it evenly with a thin layer of flour. Dissolve the chocolate in a water bath and then cool to 40 degrees. Mix the butter with half the amount of sugar until very creamy. Later, mix in the liquid chocolate and one egg yolk at a time.
In another container, beat the egg whites with salt until almost stiff, then add the remaining sugar and continue beating until a very stiff egg white is obtained. Stir about one third of the whipped cream into the chocolate mixture with a rubber spatula and then pour it over the remaining whipped egg whites. The flour is evenly distributed on top.
The dough is gently worked with the rubber spatula until it no longer has white streaks.
Spread the dough evenly with the rubber spatula on the prepared baking tray and bake in the middle of the oven for 15-18 minutes. When you pierce the dough with a fork at the end, it should not stick to the needle. It should come off easily from the edges. Immediately after baking, remove the cake from the baking sheet with a sharp kitchen knife and then place it on a roasting rack to cool.
To prepare the filling, heat the whipped cream with the chocolate in a 1-liter purée over medium heat, stirring continuously until the chocolate is dissolved. M