Toast the sesame seeds in a small pan over medium heat and shake occasionally for 3–5 minutes or until golden brown. Set aside.
Fold the tofu skin into a quadrant and trim the sides to create 4 square pieces.
Bring a large saucepan of water to a boil over high heat, add the spinach, and blanch for 30 seconds. Drain and rinse under cold running water. Use your hands to squeeze out excess moisture. Coarsely chop the leaves.
For the filling, combine the spinach, mushrooms, carrots (if using), porcini powder, sesame seeds, salt, and sesame oil in a bowl. Divide the filling into 4 equal portions.
Brush some egg onto a piece of tofu skin, place the filling along one edge of the rectangle. Roll the skin, tucking in the edges, then finish rolling it up.
Heat the vegetable oil in a large skillet (frying pan), add the vegetable rolls, and brown on both sides over medium low heat for 4–6 minutes, then remove from the pan and drain on paper towels. Cut each roll into 5 pieces before serving. Garnish with cilantro (coriander), if using.
Adapted from CHINA: THE COOKBOOK by Kei Lum and Diora Fong Chan (Phaidon 2016)