In a heavy non-reactive saucepan, stir together the mustard, brown sugar, vinegar and beer. Season with chilli powder and black, white and cayenne peppers. Bring to a simmer over medium-low heat, and cook for about 20 minutes. DO NOT BOIL, or you will scorch the sugar and peppers.
Mix in the Worcestershire sauce and butter. Simmer for another 15 to 20 minutes. Taste, and season with hot sauce to your liking. Pour into an airtight jar, and refrigerate overnight to allow flavours to blend. The vinegar taste may be a little strong until the sauce completely cools.