Imperial Darkness Yeast
Mash twice, using half of the grain each time. Boil the beer as long as possible. Marcus says he never boils an imperial stout for less than 6 hours. I might just boil for as long as it takes me to complete the second mash (1 hour). I think this will be ~3 hours total, which is a lot of propane. To each mash, add 8 g Calcium Carbonate, 2 grams calcium Chloride, and 1 gram Gypsum.
https://blackisbeautiful.beer/ Converting recipe for 5 gallon system