Soak shiitake in 200 ml boiling water until soft. Squeeze, mince, and keep the soaking liquid.
Stir Seasoning A into the shiitake soaking liquid; let cool to room temp.
In a bowl, add all filling ingredients (start with pork), pour in the seasoned liquid, and knead well until sticky.
Spread ⅓ of the filling in a heatproof dish. Cover the surface with wrappers. Add the rest of the filling, level it, then cover with more wrappers.
Brush the top with the soy–water mix.
Steam on high about 15 min (skillet + lid or foil works) until cooked through. Slice like a cake and serve with black vinegar + ginger.
