Acorn squash is a lovely, and seasonal, choice for this tempura, but you can use the batter to coat and fry up any kind of tempura. Try thinly sliced sweet potatoes, butternut squash, or eggplant, small cauliflower or broccoli florets, or even apples or pears.
Make the sauce: Stir together 1 cup warm water, the soy sauce, mirin, dashi powder and superfine sugar. Set aside until ready to serve.
Make the tempura: In a bowl, whisk the flour, cornstarch, 1 teaspoon salt and the seltzer to make a loose batter, adding up to 1/2 cup more flour as needed.
Fill a pot with 3 inches vegetable oil; bring to 350˚ F over medium heat. Working in batches, dip the squash wedges into the batter, allow the excess to drip off, then deep-fry until golden and crisp, 2 to 3 minutes. Transfer to paper towels to drain, then serve with the dipping sauce.