Preheat oven to 425 degrees F.
Butter a 9 by 13-inch glass baking dish, and line dish with parchment paper. Butter and flour parchment. Set dish aside.
Place unsweetened chocolate and 1 cup butter in the upper pot of a double boiler; melt over barely simmering water, stirring occasionally. Remove from heat and set aside.
In the bowl of an electric mixer fitted with the whisk attachment, combine eggs, vanilla, and sugar; beat on high speed for 10 minutes. Reduce speed to low, add chocolate mixture and beat to combine. Add flour and salt gradually, beating just to combine. Fold in semisweet chocolate chunks.
Pour batter into the prepared baking dish. Bake bars until edges just start to pull away from pan, but the center is still moist, 25 to 30 minutes. Transfer dish to a wire rack to cool. Using a serrated knife, cut bars into 24 pieces. Store brownies in an airtight container for up to 1 week.
When serving brownies to guests for dessert, you want them to look perfect. To make sure that no one gets an edge piece, trim the brownies around the edges of the pan so that the sides are even. Then take the trimmings, which are still every bit as delicious as the rest of the brownie, and cut them into small pieces. Put them in a resealable plastic bag and freeze. Later, when serving ice cream, use the crumbled brownie pieces instead of hot fudge for a quick and delectable topping.
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