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1
Position oven rack in the lower third of the oven; preheat to 350; butter and flour a 9 x 13 inch pan; set aside.
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2
In a bowl, whisk the flour, baking powder, and salt until combined; set aside.
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3
Add butter and both chocolates in the top of a double boiler set over simmering water; stir constantly, until half the butter and chocolate is melted.
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4
Remove the top of the double boiler, then continue stirring, away from the heat until the butter and chocolate are completely melted.
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5
Transfer mixture to a large mixing bowl and let cool for 10 minutes.
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6
Beat the brown sugar into the melted chocolate with an electric mixer on medium speed; continue beating until the mixture is smooth and silky, about 4 minutes.
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7
Beat in the eggs one at a time, allowing each to be thoroughly incorporated before adding the next.
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8
After beating in the third egg for 1 minute, stir in the vanilla.
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9
Using a wooden spoon or rubber spatula, stir in the flour mixture just until combined (do not beat).
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10
Spoon the batter into prepared pan, spreading it gently to corners.
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11
Bake for 25 minutes, or until a pick comes out with a few moist crumbs attached.
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12
Set pan on a wire rack to cool for at least 30 minutes.
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13
Cut brownies into 24 pieces while they are still in the pan.
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14
Carefully remove them with an offset spatula.
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15
Serve immediately or let cool completely before covering with plastic wrap for storage at room temperature.
