Prepare the nuoc cham dipping sauce by combining the warm water and sugar in a small bowl, stirring until the sugar is fully dissolved.

Whisk in the fish sauce, fresh lime juice, minced garlic, and sliced chili, then set the sauce aside at room temperature to let the complex flavors meld.

Thoroughly wash and dry the purple sprouting broccoli with paper towels, ensuring there is no moisture left on the stems so the batter adheres properly.

Heat about two inches of neutral oil in a deep pan or wok to 350 degrees Fahrenheit, testing the heat by dropping in a dab of batter to see if it sizzles instantly.

Make the tempura batter by gently whisking the egg yolk and ice-cold sparkling water together, then lightly folding in the cake flour and cornstarch without overmixing.

Working in small batches to avoid crowding, dip each broccoli spear into the cold batter to coat it lightly, then carefully lower it into the hot oil.

Fry the broccoli for 2 to 3 minutes until the batter is crisp and pale golden, then use a slotted spoon to transfer them to a wire rack to drain.

Serve the hot, crispy tempura immediately on a platter alongside the tangy nuoc cham dipping sauce for a stunning start to any meal.