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Place rack in upper third of oven and preheat to 350° F.
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Grease and flour an 8 or 9-inch square baking dish and set aside.
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In a medium bowl, whisk together flour, baking powder, and salt in a medium bowl and set aside.
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Bring about 2 inches of water to a boil in a medium saucepan.
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In a medium heatproof bowl add butter and unsweetened chocolate
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Place bowl over the top of, but not touching the simmering water and allow to melt.
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Stir to incorporate.
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Once melted, remove the bowl from the simmering pot.
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Whisk in brown and granulated sugar.
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Whisk in eggs, yolk, and vanilla.
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Add the flour mixture all at once to the chocolate mixture.
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Fold together with a spatula until well incorporated.
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Fold in chocolate chips.
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Pour batter into prepared pan and bake for 25-30 minutes.
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Bake until a skewer inserted in the center comes out clean.
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Remove from oven and cool completely.