I do alot of competition cooking and Bark is a must.The simplist way to achieve a bark on pork butt is to have enough sugar, i use turbinado, mixed in with your rub.We use a mustard slather on meat then the rub.Guys like D.r bbq in there books make there rub, say a cup of dry ingredients and add another quarter cup of sugar(turbinado, as it has the highest non burn temp of sugar) and blend this togeteher and apply to outside of meat.Lots of sugar to mix with fat layer.If you keep smoker 250 and below no chance of burning.We never foil our butts for comps and our cook times,low and slow, are 1-1half hours pre pound, like a slow cooked brisket at 225.We cook fat side up, non trimmed, 235-250 ,and that will melt the outside layer of fat and rub combined into a bark.The 7-9 pound boston butts are our standard and we do this for personal eating as well.I cook boston butts sometimes at 300 plus and you add the sugar near end of cook at lower temp, so you dont burn the sugar, similiar to the folks here who use bourbon etc in there rib glazes etc.This will not create the low and slow bark ,but some times i like butt in say4-5 hours.