Cook the rice by rinsing it in a fine mesh colander under hot water until the water runs clear. Drain well, then cook according to package directions. Once cooked, let the rice steam and rest for 5–10 minutes.

Prepare the teriyaki sauce by whisking together brown sugar, soy sauce, mirin, and sesame oil in a small bowl. Microwave for 30 seconds to 1 minute until the sugar dissolves, stirring to combine. Let cool slightly.

Heat a large nonstick skillet over medium heat and lightly spray with cooking spray. Arrange the sliced Spam in a single layer. Cook for 4–5 minutes until lightly browned, then flip. Reduce heat to medium-low and spoon about 1 tablespoon of teriyaki sauce over each slice. Continue cooking, flipping and spooning sauce 1–2 more times, until deeply browned and glazed. Remove from heat.

Place a sheet of nori shiny-side down on plastic wrap. Position the musubi mold in the center.

Add rice to the mold, filling almost to the top. Gently level it without pressing too firmly. Sprinkle 1–2 tablespoons of furikake over the rice. Place two slices of glazed Spam side by side on top.

Press down firmly with the mold lid to compact slightly. Carefully remove the mold.

Fold the near edge of nori up and over the Spam. Lightly brush the far edge with water and roll tightly to seal. Place seam-side down and let it sit to tighten.

Run a sharp knife under warm water and slice the double musubi in half crosswise. Repeat with remaining ingredients. The final musubi will use one slice of Spam and half a sheet of nori.

Serve immediately or wrap tightly for later.