For tempura batter, whisk flours, baking powder and iced water (about 450ml) in a large bowl to form a batter the consistency of pouring cream.
Remove stamens from inside each flower by gently opening flower and carefully snapping off stamen. Heat enough oil in a medium saucepan to reach a depth of 5cm, to 180°C. Dust flowers in extra flour then dip, one at a time, into batter and deep-fry, turning halfway, until golden and crisp (4 minutes). Drain on paper towel and season with salt. Serve with lemon wedges and mayonnaise.
