Burnt bacon mound is the result of accidentally burning a raw bacon mound while attempting to make a bacon mound. Cooking a bacon heap requires 54 Cooking, and burning it will become less common with a higher Cooking level. It appears that the burnability of bacon in general is based on the lowest-level raw bacon, which requires only 14 Cooking. This means that the bacon mound is much easier to cook successfully than would be expected just based on its level, so the burnt version is relatively hard to obtain even at the minimum level.