Chop chocolate into about 1/2-inch chunks; you should have about 2 cups. In a small, microwave-safe bowl, combine half the chocolate and the butter. Heat in a microwave oven on half-power (50%) just until chocolate is soft and butter is melted, 1 to 1 1/2 minutes. Stir until mixture is smooth. Let stand until just warm to touch.
In a bowl, with a wooden spoon, beat eggs, sugar, salt, and vanilla until smooth. Add chocolate mixture and stir until well blended. Add flour, about a third at a time, stirring after each addition just until blended. Add remaining chopped chocolate and mix just until chunks are evenly distributed.
Line bottom and sides of a 9-inch square baking pan with cooking parchment, draping over rim a little. Scrape batter into pan; spread level.
Bake in a 325° regular or 300° convection oven just until surface develops a thin crust (like the delicate layer of ice that forms on freezing water) and a fingertip pressed very gently in the center leaves a soft impression, 20 to 25 minutes; take care not to overbake.
Cool completely in pan on a rack, at least 1 hour. Lift brownie out on parchment, peel off parchment, and set brownie on a board. Cut into 8 squares or wedges or 16 triangles (see notes).