Melt butter in a small pan. Blend in miso and sugar until incorporated.
Pour miso butter over pumpkin and mix with a spatula. Pumpkin cubes should be well coated with miso butter.
Heat a large non-stick fry pan over medium heat. Place a layer of pumpkin in the pan and fry for about 5 minutes. Flip the cubes over and continue to fry for another 5 minutes. Fry in two batches if necessary.
Transfer pan fried pumpkin to a dish and sprinkle with toasted sesame seeds.