In a medium saucepan, over medium-low heat melt the butter. Cook the onions until soft and translucent, 12 to 15 minutes. Stir in the garlic and tomato and Achiote paste and cook an additional 3 to 5 minutes, stirring frequently. Add the chicken stock, salt and black pepper. Bring to a boil, reduce to a simmer and cook, stirring and skimming frequently, 12 to 15 minutes. Add the vinegar and cook at end for 2 to 3 minutes. Puree in a blender or food processor. Store in refrigerator 2 to 3 days.
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