Preheat the oven: Preheat your oven to 350°F (175°C). Grease a soufflé dish with butter and set aside.

Mix the pumpkin base: In a large bowl, combine the pumpkin puree, milk, sugar, egg yolks, vanilla extract, cinnamon, nutmeg, and salt. Whisk until the mixture is smooth and well combined.

Beat the egg whites: In another bowl, beat the egg whites until stiff peaks form. This will help give the soufflé its light and airy texture.

Combine mixtures: Gently fold the beaten egg whites into the pumpkin mixture. Be careful not to deflate the egg whites; use a spatula to combine until just incorporated.

Pour and bake: Pour the mixture into the prepared soufflé dish and place it in the preheated oven. Bake for 30-35 minutes, or until the soufflé is puffed and golden brown.

Cool and serve: Once baked, allow the soufflé to cool for about 10-15 minutes before serving. Dust with powdered sugar if desired.