Chop off ends of squash, cut in half lengthwise and then into 2-inch pieces. Place in a steamer basket and place basket in a pot over simmering water. Place a lid on the pot, turn heat to low and steam squash for 12-15 minutes or until squash is very tender.
Place squash in a medium bowl and mash well with a fork.
In a large bowl, combine cornmeal, flour, sugar, ½ teaspoon salt, black pepper, and cayenne pepper. Mix together .
To bowl with squash, add onion, jalapeno, buttermilk, and egg. Mix together well. Add squash mixture to cornmeal mixture and stir just until blended.
Pour oil to a depth of 1 inch in a deep cast iron skillet or a Dutch oven. Heat oil to 350 degrees. Drop batter by rounded tablespoonfuls into oil and fry until golden brown on each side, about 3 minutes per side. Drain on paper towels and sprinkle with remaining ½ teaspoon salt while still hot.