Whisk flour, sugar (if using), and salt in a bowl. Cut in cold butter until crumbly.
Add ice water, a tablespoon at a time, mixing just until dough comes together. Shape into a disk, wrap, and chill for 30 minutes.
Preheat oven to 400°F (200°C). Pierce potatoes and roast for 45–60 minutes until tender. Let cool, peel, and mash. Measure out 1½ cups.
Reduce oven to 350°F (175°C). Mix mashed potatoes with both sugars, milk, butter, egg, vanilla, cinnamon, nutmeg, and salt until smooth and creamy.
Roll chilled dough to ⅛-inch thick. Cut circles slightly larger than muffin tin wells. Press into greased tin and prick bottoms with a fork.
Spoon filling into each crust, about ¾ full. Bake for 20–25 minutes until set and slightly puffed. They’ll settle as they cool.
Cool in the pan for 10 minutes, then transfer to a rack. Garnish with whipped cream, maple syrup, or nuts if desired.