Preheat your oven to 325°F. Line an 8x8-inch metal baking pan with parchment paper, leaving a 2-inch overhang at each side.
Melt the chocolate and butter together. You can do this several ways: Place chocolate and butter in a stainless steel saucepan over very low heat stirring constantly until melted and smooth, use a double boiler, or melt it in the microwave at 50% power with frequent stirring.
In a separate bowl, combine eggs with both sugars and whisk vigorously until pale and thick. Mix in vanilla and salt. Add the warm chocolate mixture and whisk until evenly incorporated.
Sift the flour and cocoa powder over the batter and fold it through until it is evenly combined.
Scrape the mixture into your prepared pan and spread it out evenly. Bake for 28-32 minutes until puffed and shiny on top and a skewer inserted into the center comes out with a few moist, sticky crumbs attached.
Transfer pan to a wire rack and let cool completely in the pan. For best results, refrigerate for 1-2 hours before slicing.
