Preheat oven to 350. Butter a 9 x 13 inch baking pan. Line the bottom and up two sides with a single sheet of parchment paper, letting the paper overhang the two long sides of the pan by an inch or so.
Place a large pot filled with an inch or two of water over low heat and bring to a gentle simmer. Combine the chocolate and diced butter in a stainless steel or glass bowl. Set the bowl over the pot. Stir occasionally until melted. Remove bowl from the top of the pot and set aside to cool slightly.
Meanwhile, in a large bowl, beat the eggs just until the yolks and whites are blended. Whisk in the sugar and beat until the mixture is creamed together (slightly thickened and pale). Stir in the vanilla.
Sift together the flour, cocoa and salt. Set aside.
Pour the chocolate into the egg and sugar mixture, stir to blend well. Add the flour mixture in three additions, using a spatula to gently fold in each addition before adding the next.
Scrape the batter into the pan and place in the center of the oven. Bake for approximately 35 minutes, or until a tester inserted in the center of the brownies comes out clean. Transfer pan to wire rack and let cool completely before cutting. Use the parchment overhang to lift from pan.