Preheat oven to 350 degrees and butter and flour an 8-inch square baking pan, knocking out excess flour. In a heavy 1 1/2-quart saucepan melt bittersweet and unsweetened chocolate and butter over low heat, stirring, until smooth and remove pan from heat. Cool mixture to lukewarm and whisk in sugar and vanilla. Add eggs, 1 at a time, whisking well until mixture is glossy and smooth. Stir in salt and flour just until combined and stir in chocolate chips.
Spread batter evenly in pan and bake in middle of oven 25 to 30 minutes, or until a tester comes out with crumbs adhering to it. Cool brownies completely in pan on a rack before cutting into 16 squares. Brownies keep, layered between sheets of waxed paper in an airtight container at cool room temperature, 5 days.