Cook two cups of sushi rice and add vinegar and sugar mixture.
In a bowl, mix together the tuna, Kewpie, and sriracha. Add more hot sauce or Kewpie if needed.
Cover your sushi mat with cling wrap to keep your mat clean.
Take your one sheet of nori and place it with the shiny side face down.
Take ¼ cup of rice, and using your hands, layer the rice evenly on the sheet. Dip your hands in the water bowl to keep your hands from sticking to the rice. I try to make a horizontal indent on the rice to make room for the spicy tuna mix.
Add one scoop of the spicy tuna mix horizontally on the bed of rice, leaving about half an inch of room from the bottom of the nori. The key is to add less fish than you think.
Take the bottom edge of the sushi mat and slowly move the nori sheet over the filling, completely wrapping the spicy tuna and rice in the sheet. Use an upwards motions with the sushi mat to guide your spicy tuna roll upwards. Try to keep the roll tight by applying even and gentle pressure while rolling upwards. Be gentle to prevent your nori from breaking or having rice or fish from coming out the sides.
Continue this motion until your roll comes away from the mat. You may need to cut the additional nori at the end.
Using a very sharp knife, cut the roll in quick and steady slices. Wipe down the knife in between cuts to prevent mangling your roll.