Whisk the sake kasu, 1/4 cup of water, the sugar and salt together in a medium bowl until smooth. Place the salmon collars in an airtight container and coat them with the sake kasu marinade. Refrigerate the collars overnight or for up to 2 days.
For grilling the collars:
Heat a charcoal or gas grill to high heat. Remove the collars from the marinade and wipe dry. Use tongs and a folded paper towel to grease the grill grates with the vegetable oil.
Lay the collars on the grill skin-side down and cook until the edges begin to caramelize and the flesh of the salmon becomes pale pink, about 1 1/2 minutes. Flip the collars over and cook on the second side until the flesh easily pulls away from the bone, about 1 1/2 minutes longer, then transfer the grilled collars to a platter. Rest the collars for 2 minutes before serving with your favorite pickled vegetables and some cold sake.
Sake kasu or simply kasu is the solid white matter that is left over after the rice mash is pressed and fermented during the sake making process. It is available through Artisan Sake Maker.
Salmon collars are a natural byproduct of filleting salmon, meaning your local fishmonger or seafood counter is your best bet for finding them. Call ahead so the collars will be waiting for your arrival.