Melt the butter in a saucepan, then pour it over the Dark Chocolate

Whisk Eggs and Sugar together just so it becomes frothy, no need for any sort of soft/medium/hard peaks.

Butter and Chocolate mixture needs to cool down below 50°C, which should be very soon but good to check just in case.

When Butter and Chocolate mixture has reached a cool enough temperature, pour it into the Egg and Sugar mixture while folding it together. Stop folding when you've emptied the Butter-Chocolate bowl.

Sift the Flour and Salt into the mix while folding. Keep folding until the dough is even, but try not to overdo it as it could deflate the dough.

Pour the batter into a greased and floured baking tin and bake at 200°C for about 25 minutes.