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