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Prepare a medium-size bowl of ice water.
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In the bowl of an electric stand mixer fitted with the whisk attachment, gradually whip the cream and sour cream together.
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Increase the speed of the mixer and continue whipping until the cream separates and the mixture thickens.
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Use a rubber spatula to gather up the butter and remove it from the bowl.
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There will be some liquid that is a natural result of this process.
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That liquid is actually buttermilk.
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Gather the ball of butter together into a double layer of cheesecloth or a thin kitchen towel and squeeze the cloth to remove the remainer of the buttermilk.
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Plunge the ball of butter (still in the cloth) into the ice bath to wash any buttermilk off the surface.
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Sprinkly with salt and pepper, to taste.
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Pack the butter into a bowl or roll it into a ball or log shape using plastic wrap.
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Use immediately or store in the refrigerator.