Preheat the oven to 180 °C and place the chocolate and butter in an ovenproof dish to melt gently in the warming oven.
Once melted, remove and set aside to cool slightly.
In a separate bowl, whisk the egg yolks with sugar and a pinch of salt until the mixture becomes pale and frothy.
In another clean bowl, beat the egg whites into firm peaks.
Combine the cooled chocolate-butter mixture with the yolk-sugar mixture, then sift in the flour and mix gently.
Add a small amount of the whipped egg whites to loosen the batter, then carefully fold in the rest, ensuring no streaks of white remain.
Pour the batter into the prepared cake tin and smooth the surface with a spatula.
Bake on the middle shelf of the oven for 25 minutes.
Though some prefer 35 minutes, a slightly gooey centre is ideal—check with a knife; it should come out with a smudge of batter.
Once the cake is just cool enough to handle, turn it out of the tin.
Dust with icing sugar and serve with cream, ice cream, or fresh fruit.