Adjust oven rack to bottom third level and preheat oven to 375°F (190°C). Brush 2.5L soufflé dish or casserole dish with butter; sprinkle with 2 tbsp (30 mL) of the granulated sugar, tapping out excess. Wrap parchment paper strip around outside of dish to extend 2-inches (5 cm) above rim; tie with kitchen string.
Whisk milk, starch, spice, vanilla, cinnamon and salt in medium saucepan. Bring to a low boil over medium heat, whisking constantly. Whisk in pumpkin. Transfer to large bowl and let cool to room temperature.
In stand mixer, whip egg whites on medium speed until foamy. Sprinkle in remaining ½ cup (125 mL) granulated sugar with mixer running, 1 spoonful at a time until stiff, glossy peaks form, about 3 minutes. Fold one-quarter of egg whites into pumpkin mixture; fold in remaining whites until no streaks remain. Scrape into prepared dish.
Bake until edge is firm, top is lightly brown and centre is jiggly, about 30 minutes. Remove paper; sift icing sugar over top and serve immediately.
To serve: Make a hole in centre of soufflé with spoon, drizzle in crème anglaise or custard.
