Put the tofu in a shallow heatproof bowl and place the bowl in a bamboo steamer over a pot or a wok of rapidly boiling water. Cover the steamer with the lid and steam the tofu for 5 minutes to heat through.
Carefully remove the bowl from the steamer and pour off any excess liquid. Cut the tofu into six pieces, spoon the XO sauce over the tofu, then drizzle with soy sauce and sprinkle with the spring onions and sugar. Heat the peanut oil in a small pot until just smoking, then carefully pour the hot oil over the tofu to serve.