Pour off all but 2 tablespoons of the bacon fat from the skillet. Add the butter and stir in the leeks. Cover the skillet with a sheet of wax paper and a tight-fitting lid and cook the leeks over moderate heat, stirring once or twice, until they are tender, about 8 minutes. Uncover the skillet, increase the heat to moderately high and cook the leeks until they are lightly browned, about 2 minutes. Add the crème fraîche, season with salt and pepper and cook over low heat until creamy and thickened, 5 to 6 minutes longer. Stir in the bacon.