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.
Add the remaining kosher salt, black pepper, the onion powder, celery seed and paprika. Stir to blend together. Check for seasoning, adding more to suit your taste.
Whisk together the egg and melted butter with the milk and add to the squash.
Combine the flour and baking powder and stir into the squash and 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.
Spoon the squash batter onto the hot griddle and cook until brown and the top is set before flipping, about 5 minutes. Continue cooking until browned on both sides and the edges appear dry.
Serve with a dollop of sour cream and chopped scallions!