Boil potatoes: Place potatoes in a large pot, cover with cold water, and bring to a boil. Cook until tender (easily pierced with a fork), about 15-20 minutes.

Drain and mash: Drain potatoes thoroughly. Return to the pot and mash until completely smooth and creamy using a potato masher or mixer.

Cook sugar and butter: In a saucepan, combine sugar and butter over medium heat. Stir constantly until butter melts and sugar dissolves. Bring to a gentle simmer, stirring frequently to prevent burning.

Combine: Pour the sugar-butter mixture into the mashed potatoes. Stir until well combined and smooth. The mixture will thicken.

Add flavorings: Stir in vanilla and salt. Add optional nuts and marshmallows, mixing gently.

Shape and cool: Grease a baking sheet. Drop spoonfuls of the mixture onto the sheet, shaping into mounds or squares, leaving space between each.

Cool completely: Let the potato candy cool completely at room temperature to firm up before serving.