Place the squash in a large pan with 2 teaspoons of the kosher salt and cover with water. Bring the water to a boil over medium heat. Cook partially covered until the squash is tender and cooked through, about 30 to 40 minutes.
Pour the squash out into a colander to drain. Using a potato masher, break up the squash into small pieces. Allow the squash to drain removing as much of the water as possible.
Transfer the squash to a large mixing bowl – you should have about 3 cups of squash. Add the chopped onion and parsley.
Toss in the remaining kosher salt, black pepper, onion powder, celery seed and paprika. Stir to blend together. Check for seasoning, adding more to suit your taste.
In a separate container whisk together the egg, melted butter and milk together; add to the squash.
Whisk the flour and baking powder together then stir into the squash. Let the batter rest for 10 minutes.
Heat a griddle or large nonstick skillet over medium heat. (350-degrees if using a temperature controlled griddle.)
Scoop the squash batter onto the hot griddle. smoothing out so that the pancakes are about ½" in height. Cook until brown and the top is set before flipping, about 10 minutes. Continue cooking until browned on both sides and the edges appear dry, about an additional 10 minutes.
Serve with a dollop of sour cream and chopped scallions.