1. Preheat the oven to 350°. Grease a 9 × 13-inch baking pan, preferably aluminum. Line the bottom and up two sides with a single sheet of parchment paper, letting the paper overhang the two long sides by an inch or so. Not only will this prevent any sticking, but it will also make removing the cooled brownies from the pan easy and neat.
2. Place a large pot filled with an inch or two of water over low heat and bring the water to a very gentle simmer. Combine the two chocolates and the diced butter in a stainless steel or glass howl and set the howl over the pot. Stir the chocolate occasionally until it is about ¾ melted, then turn off the heat under the pot and stir until the mixture is completely melted and smooth. Remove the bowl from the top of the pot and set it aside to cool slightly.
3. Meanwhile, in a large bowl, beat the eggs slightly just to blend the yolks and the whites. Whisk in the sugar and beat until the mixture is thickened and pale, about 2 minutes if beating by hand, 1 minute if using beaters or a stand mixer. Stir in the vanilla. In a small bowl, sift the flour, cocoa and salt together.
4. When the chocolate is just warm but not hot, pour it into the egg and sugar mixture, stirring to blend well. Sift the flour mixture over the batter in three additions, using a rubber spatula or a wooden spoon to gently fold in each addition before adding the next.
5. Scrape the batter into the pan and place it in the centre of the oven. Bake for approximately 35 minutes, or until a tester inserted in the centre of the brownies comes out with a few moist crumbs clinging to it, and the surface is set, shiny, and perhaps beginning to crack slightly at the edges. Transfer the pan to a wire rack and cool completely before cutting into bars. Don’t overbake these brownies! A gooey, chewy texture is part of their allure. The whole cake can be lifted out of the pan onto a cutting surface using the overhang of parchment paper, or you can cut the bars right in the pan. A thin-bladed knife dipped in hot water and wiped dry between cuts makes cutting these sticky, gooey bars much easier! Makes enough for, oh, three or four chocoholics, or several more mere mortals. Store brownies well wrapped in plastic, at room temperature for gooier brownies, or in the refrigerator for denser, fudgier bars, for up to 5 days (they’ll never last that long). I find keeping the leftover bars in the pan and covering the whole pan with wrap keeps the brownies fresh and soft.