Place vinegar, sugar, ketchup, butter, hot pepper sauce, lemon juice, Worcestershire sauce, red pepper flakes, mustard powder, salt, and pepper into a saucepan. Bring to a simmer over high heat, then reduce heat to low, and simmer, stirring occasionally, for 30 minutes. Remove from the stove and pour into a heat-proof bowl. Cover and refrigerate the sauce for 2 days. It's worth the wait!!