In a large bowl over a pot with ½-inch of simmering water, stir together chocolate and butter until completely melted, smooth and glossy. You can also melt it in the microwave with 20 second bursts and stirring each time. Set it aside and keep it warm (no need to let it cool).
Combine eggs and sugar in a large bowl and beat with a wire whisk or a handheld mixer on medium-high speed until pale, thick and a bit fluffy. It will take about 2 minutes. I like to do this by hand with a whisk. Just whisk as vigorously as you can for 1-2 minutes until the mixture is lightened in color and doubled in volume. When you lift up the beaters or the whisk, the egg mixture will form a slowly disappearing ribbon on the surface of the mixture when it falls back into the bowl.
Mix in vanilla and salt. Add the warm chocolate mixture and mix well with a spatula or the whisk until evenly combined. The mixture should tighten up and become glossy when the chocolate is thoroughly incorporated.
Sift in flour to remove lumps and fold it in gently until well incorporated and smooth.
Scrape batter into prepared pan and use a small offset spatula to smooth out the surface. Bake for 20-25 minutes until slightly puffed and a toothpick inserted someplace between the edge and the center comes out with a few moist crumbs attached. Do not over-bake!
Transfer pan to a wire rack and let cool completely before slicing. Refrigerate for an hour after cooling and slice with a hot dry knife for the cleanest slices.