Combine the cucumber and carrot in a colander set over a bowl. Toss with the salt and let sit for at least 10 minutes to draw out excess moisture.

Squeeze the vegetables firmly to remove liquid, rinse under cold water to remove the salt, and squeeze once more until dry.

Whisk the rice vinegar, sugar, and sesame seeds in a medium bowl until the sugar dissolves. Add the drained vegetables and toss to coat; the pickles will improve if they sit while you finish the rest of the dish.

Prepare your steamer over a wok or skillet with enough water to reach the bottom edges. If using a metal lid, wrap it in a kitchen towel to prevent condensation from dripping onto the buns and making them soggy.

Steam the buns over boiling water for 8 to 10 minutes. Turn off the heat but leave the lid on for an additional 10 minutes—opening it too soon can cause the buns to collapse from the sudden temperature change.

Transfer the buns to a plate and cover with a clean, damp towel to keep them soft and pliable.

Carefully open each bun and stuff with a piece of char siu tofu, followed by a generous portion of the pickled vegetables. Top with chopped peanuts and red chiles, then serve warm while the buns are at their fluffiest.