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1
Preheat the oven to 325 degrees F. Butter an 8-inch square baking dish with 1 teaspoon butter.
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2
Cut a piece of parchment paper in a strip to line the bottom and 2 sides of the dish.
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3
Butter the paper with the remaining 1/2 teaspoon butter and set aside.
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4
Place the unsweetened chocolate and 8 tablespoons butter in a medium heat-proof bowl that fits comfortably over a saucepan filled with 1 inch water.
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5
Place the double-boiler over medium heat and cook, stirring constantly with a silicone spatula, until the chocolate and butter are just barely melted.
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6
Remove the bowl from the heat and continue to stir for 1 minute to cool slightly.
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7
The mixture should be warm to the touch, not hot.
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8
Using a hand-held mixer, beat the sugar, vanilla and salt into the chocolate mixture until smooth.
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9
Add the eggs and yolk all at once and beat until fully incorporated and smooth.
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10
Fold in the flour and semisweet chocolate chips with a silicone spatula until just barely combined; a few streaks are fine.
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11
Pour the mixture into the prepared pan and bake on the middle rack until just set and a toothpick inserted in the center comes out clean, about 30 minutes.
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12
Do not overbake.
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13
Allow to cool to room temperature before cutting into 16 small pieces.
