Preheat oven to 350F. Cut each of the nori sheets into 4 pieces and place the nori squares in a muffin tin. In a separate large bowl, mix rice with sugar, salt, sesame oil, and rice vinegar.
Spread all of the rice on top of the nori sheets in muffin slots, evenly dividing it among all of the individual muffin compartments.
In a separate bowl, mix chopped imitation crab, ketchup, and mayonnaise.
You can swap the ketchup and regular mayo for kewpie mayo or spicy Japanese mayonnaise.