In a small saucepan, combine soy sauce, ginger, sugar and cilantro. Place the saucepan over medium-high heat and simmer until liquid is reduced by half.
Reduce the heat to low and slowly stir in the cold Plugrá Butter piece by piece.
Strain the butter and keep warm for service.
Place the oysters in a bamboo steamer; you will probably need to use both baskets.
Place the steamer over a pot of boiling water, making sure the pot is wider than the bamboo basket and only has a half inch of water in it.
Steam the oysters for 4 minutes and check to see if they have started to open; if not, steam another minute or two until all are open. Discard any oysters that do not open.
Remove from heat. Using a dry towel and an oyster knife, remove the top (flat-side) shell of the oyster. Using the oyster knife, detach the oyster from the bottom and leave it in the shell. Repeat with the remaining oysters.
Place coarse rock/sea salt on eight serving plates to stabilize oyster shells. Arrange 3 oysters on each plate.
Top with Ginger-Soy Butter and garnish with sliced scallions.