Whisk 1 cup cake flour and 1/2 teaspoon kosher salt. Whisk in 1 cup cold seltzer. Dip 1 bunch trimmed asparagus in the batter and fry in batches in 360 degrees F vegetable oil until crisp, about 2 minutes (drop them in one at a time to prevent sticking). Remove and drain on a rack; season with salt. Serve with ponzu sauce for dipping.