Melt the butter in a large 12-16 quart stockpot. Add the onions, celery, carrots, bay leaves, thyme, parsley and peppercorns, and cook over moderate heat, stirring occasionally, until the vegetables soften without browning. This will take up to 10-15 minutes.
Heat a 4-quart heavy pot over low heat, and add the bacon and butter. Cook the bacon slowly so that some of the fat renders, then increase heat and cook bacon until brown and crispy. Remove the bacon with a slotted spoon,