In a small nonreactive saucepan, heat the tomato sauce, vinegar, and shallots until they form a thick paste, measuring about 1/4 cup. Now slowly whisk in the cold butter, a few cubes at a time into the reduced sauce. Make sure to completely incorporate each addition of butter before adding any more. Once all the butter is incorporated, stir in the parsley and adjust the seasonings. Serve atop potato pancakes, fish, or chicken.
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