1. Heat oven to 375°. On a lightly floured surface, roll dough into a 12″ round. Fit into a 9″ pie plate. Trim edges and crimp; chill 30 minutes.
2. Whisk sugars and eggs in a bowl until pale and fluffy. Add pumpkin, cream, syrup, potato starch, cinnamon, ginger, nutmeg, clove, and salt; whisk until smooth. Pour filling over dough; using a spatula, spread into an even layer. Bake until just set in the center, 45–50 minutes. Transfer pie to a rack; let cool to room temperature, then refrigerate until cold, about 1 hour.
3. Sprinkle Demerara sugar evenly over surface of pie. Guide the flame of a blowtorch back and forth over surface until sugar caramelizes. Serve immediately.