Pour vegetable oil into a large pot fitted with a deep-fry thermometer to come 2" up sides; heat over medium-high until thermometer registers 375°.
Meanwhile, whisk flour, baking powder, kosher salt, and ½ cup cornstarch in a medium bowl. Add club soda and whisk just to combine.
Working in batches, dust squash and scallions with some cornstarch, shaking off excess, then dip in batter, letting excess drip off. Fry, turning occasionally, until golden, about 4 minutes. Drain on paper towels. Serve drizzled with honey and sprinkled with red pepper flakes and sea salt.
