In a large bowl, mix the flour, cornstarch, baking powder, and salt. Add the egg yolk and club soda last. Whisk to form a smooth batter.
Preheat the peanut oil to 375 degrees in a heavy pot or deep fryer.
Dip the vegetables in the batter, shaking off excess. Then fry in small batches for about 2-3 minutes until puffy, golden, and cooked through. Drain on paper towels.
Sprinkle with toasted sesame seeds. Serve the tempura with Wasabi Garlic Mayonnaise for dipping.