Melt the butter in a large casserole over low heat. Add the onions, add the salt and pepper and stir to coat well. Cook very slowly over slow heat until the onions are soft and translucent. In rapid succession add the chili powder and stir to incorporate fully. Stir in the tomato paste and sugar and moisten with the vinegar. Continue to cook over low heat. As the liquids begin to bubble, be careful not to burn the tomato and sugar. Nothing should caramelize in this dish. Add the bourbon carefully, allowing the alcohol to burn off. Add the broth and braise, with a lid on, until the onions are nearly dry.