In a mixing bowl, combine the leftover mashed potatoes with the sweet rice flour, tapioca starch, salt, and sugar. Mix until you have a smooth, cohesive dough.

Pinch off small pieces of the dough and roll them into balls about the size of a golf ball. Flatten each one slightly to form a patty and add any optional mix-ins at this stage.

Heat a generous amount of oil in a skillet over medium heat. Once shimmering, gently place the patties in the skillet, frying each side for about 3–4 minutes, or until golden-brown and crispy.

Transfer cooked mochi to a plate lined with paper towels to absorb excess oil. Repeat until all patties are cooked.