Scrub the potatoes and place them whole, unpeeled, in a large pot of water. Bring to a boil and cook until fully tender when pierced, 25β35 minutes. Drain well, return to the warm pot for 1β2 minutes to steam off excess moisture, then peel while still warm.
Rice the warm potatoes into a large bowl. Add the unsalted butter, heavy cream, granulated sugar, and kosher salt. Stir until smooth and evenly combined, then press into a flat layer. Cover and chill until completely cold, at least 4 hours or overnight.
Break up the chilled potato mixture, then sprinkle in the all-purpose flour (1.25 cups). Fold and knead gently just until a soft, cohesive dough forms that is barely tacky; avoid overworking.
Preheat a lefse griddle or cast-iron griddle to 450Β°F/230Β°C. Lightly flour a cloth-covered board and rolling pin with all-purpose flour (for dusting). Divide the dough into 12 equal pieces and roll each into a ball; keep covered with a towel to prevent drying.
Working with one piece at a time, roll the dough very thin (about 1/16 inch/1β2 mm), adding a whisper of flour as needed to prevent sticking. Use a lefse stick or long spatula to lift the round.
Lay the lefse onto the hot griddle. Cook until light brown freckles appear and the surface looks dry in spots, 30β60 seconds, then flip and cook the second side 20β40 seconds until flexible with scattered brown spots. Adjust heat to prevent scorching. Stack cooked lefse between clean kitchen towels to keep soft. Repeat with remaining dough.
Serve warm or at room temperature, spread with unsalted butter (for serving) and sprinkled with granulated sugar (for serving) and ground cinnamon (for serving), if desired. Cool completely before storing; wrap well and refrigerate up to 3 days or freeze up to 2 months.