Boil potatoes: Place potatoes in a large pot, cover with cold water, and bring to a boil. Cook until very 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 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.

Combine: Pour the sugar-butter mixture into the mashed potatoes. Stir until completely smooth and slightly thickened.

Add flavorings: Stir in vanilla and salt. Add pecans and marshmallows (if using) and gently mix.

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

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