Rinse the Calrose rice under cold water until clear and cook according to package instructions, about 20 minutes.

In a bowl, whisk together dark brown sugar, soy sauce, mirin, and optional sesame oil. Heat until sugar is dissolved.

Pan-fry sliced Spam in a non-stick skillet until golden, adding teriyaki sauce in the last minute to caramelize.

Lay a sheet of plastic wrap and nori on a clean surface, placing the musubi mold on top.

Pack a layer of rice into the mold, add furikake, then top with teriyaki-glazed Spam.

Wrap the nori over the Spam and rice, sealing with a bit of water if needed. Let sit to firm up.

Slice the musubi into bite-sized pieces and serve fresh or pack for later.