Prepare the vegetables by washing and thoroughly drying them. Slice the sweet potato, zucchini, bell pepper, onion, and mushrooms, and cut the broccoli into small florets. Keep the pieces fairly thin so they cook quickly.

Make the dipping sauce by combining the vegetable broth, soy sauce, mirin, and maple syrup in a small saucepan. Bring to a gentle simmer for about 3 to 5 minutes, then remove from the heat and let it cool.

Whisk the flour, cornstarch, baking powder, and salt together in a large bowl. Slowly add the ice-cold sparkling water and gently stir until just combined. A few small lumps are fine; avoid overmixing to keep the batter light and crisp.

Heat about 2 inches of oil in a deep skillet or wok to 325–350°F (165–175°C). Pat the vegetables dry again before dipping them into the batter so the coating adheres properly.

Dip the vegetables into the cold batter one at a time, letting the excess drip off. Carefully place them into the hot oil without overcrowding the pan.

Fry the vegetables in batches for about 1 to 2 minutes per side, or until the coating is pale golden and crisp. Sweet potato slices may need 3 to 4 minutes because they take longer to cook.

Transfer the finished tempura to a wire rack to drain and stay crisp. Serve immediately with the prepared dipping sauce.