Place butternut squash in a microwave safe bowl. Add the water and cover the bowl with microwave safe plastic wrap. Microwave for 15 minutes. Let cool slightly and mash until smooth.
Preheat the oven to 350 degrees and grease a pie plate or 9 x 9 baking dish with cooking spray. Set aside.
In a large bowl, beat together the sugar, oil, eggs, milk and squash until combined. Fold in the flour until just combined.
Pour the mixture into the prepared baking dish and bake for 30 minutes, remove from oven, sprinkle with the cinnamon sugar, and return to the oven for another 30 - 40 minutes until golden brown and set.