Preheat the oven to 400°F, and set the rack in the center of the oven. 

Butter and sugar 6 eight-ounce ramekins, and set them on a cookie sheet.

Place the milk, pumpkin pie spice, egg yolks, flour, salt, and baked sweet potato in a blender. 

Blend on high until completely smooth. Then pour the mixture into a bowl.

Rinse the blender jar with soapy water, and dry thoroughly. 

Then place the egg whites, sugar, and cornstarch in the blender. 

Process on a low setting until very frothy.

Pour the frothy egg whites over the sweet potato mixture, and fold in. 

Divide evenly among the prepared ramekins, and bake for approximately 25–30 minutes, until they rise high above the ramekins.

Remove soufflés from the oven, and dust with powdered sugar. Serve immediately.