Soak the mushrooms in room temperatured water, just enough to cover. Leave for a few hours. If you're in a rush, you can try to use boiling water, but this usually ends up in really soggy mushrooms.
Once tender, drain the mushrooms and reserve the liquid in a bowl. Put mushrooms in a colander, toss in cornflour and rinse. Squeeze the mushrooms over the bowl of reserved liquid and arrange in a single layer in another dish.
Toss in extra cornflour, sugar and oil. Steam for 15 minutes. After 15 minutes, switch off the gas and leave for 5 minutes, keeping the lid on.
Heat oil in a wok or casserole. Once hot, add ginger, then mushrooms. Top with reserved liquid, oyster sauce and pepper powder. Stir thoroughly, cover and simmer for 20-30 minutes. Taste sauce. If it lacks flavour, add some more oyster sauce.
They taste good fresh, but even better after a few days.