Boil potatoes until fork tender
Use a ricer or sieve to mash potatoes until smooth with no lumps remaining
Add butter and milk to mashed potatoes and stir until mixture is on the dry side
Form mixture into patties and add a strip of seaweed if using
Chill patties for 20 minutes
In a small saucepan, combine soy sauce, mirin, sugar or honey, and water
Heat sauce until sugar is dissolved
Heat 2 tbsp oil in a separate pan on medium-high heat
Fry patties on both sides until golden brown, about 1-2 minutes each side
Remove from heat and add sauce and remaining butter to coat each mochi
Carefully transfer to a plate and serve
