Heat butter in small nonreactive saucepan over medium heat until foaming; add onion and cook, stirring occasionally, until softened, 2 to 3 minutes. Add garlic and chili powder; cook, stirring constantly, until fragrant, about 20 seconds. Add tomato juice, 1/2 cup vinegar, Worcestershire sauce, mustard, chipotle, molasses, and salt; increase heat to high and bring to simmer, then reduce heat to medium and continue to simmer, stirring occasionally, until sauce is slightly thickened and reduced to 1 1/2 cups, 30 to 40 minutes. Off heat, stir in pepper and remaining 1/4 cup vinegar. Cool to room temperature before serving. (Can be refrigerated in airtight container for up to 4 days; bring to room temperature before serving.)