Preheat the oven to 350 degrees F.
Fill a large, microwave-safe dish with 1/2 inch of water. Place squash into the dish and cover. Microwave for about 8 minutes and then drain (the squash should be tender enough to mash, but not overcooked).
With a potato masher, food processor, or fork, mash the squash thoroughly, but do not puree (the squash should be pulpy). Add soymilk, syrup, sweetener, salt, egg substitute, vanilla extract, and cinnamon. Mix the ingredients thoroughly, but do not over-stir.
Cook's Note: Adding the ingredients in this order gives the squash enough time to cool before adding the egg substitute, which could begin to "cook" if added too early.
Transfer to an 8-inch by 10-inch baking pan sprayed with nonstick spray. Bake in the oven until mostly firm, 45 to 50 minutes.
Top with mini marshmallows. Return to the oven until the marshmallows begin to brown, about 5 minutes. Allow to cool before serving!
PER SERVING (1/8th of casserole): 142 calories, <0.5g fat, 263mg sodium, 33g carbs, 3g fiber, 12.5g sugars, 5g protein