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Preheat the oven to 350 degrees F.
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Lightly grease a 9" x 13" baking pan and line the bottom with parchment paper.
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In a large bowl, whisk half of the milk with the cocoa to make a smooth paste.
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In a saucepan on medium heat, heat the remaining milk just until steaming.
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Whisk into the cocoa mixture, then whisk in the butter until melted.
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Let cool slightly.
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Whisk in the eggs, sugar, sour cream and vanilla until blended.
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Sift the flour, baking soda, baking powder and salt over the cocoa mixture.
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Stir just until moistened.
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Spread in the prepared pan.
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Bake for about 40 minutes or until the tester inserted in the centre comes out clean.
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Let cool in the pan on a rack.