Cook the sushi rice in a rice cooker. Once done, transfer to a bowl and let it cool slightly.
Dice the Spam into small cubes.
In a bowl, mix together the soy sauce, sugar, mirin, and oyster sauce until the sugar dissolves.
Heat a pan over medium heat and add the diced Spam. Cook until lightly browned.
Pour the sauce over the Spam and reduce the heat to low. Continue cooking until the mixture thickens and the Spam becomes glossy and glazed.
Preheat your oven to 350°F (175°C).
Line each muffin cup with a piece of nori.
Add a spoonful of sushi rice into each nori-lined cup.
Top with the glazed Spam.
Bake for 12–15 minutes.
Drizzle with Kewpie mayo.
Remove from the oven and enjoy!