In a large bowl, dissolve 2 Tbs. of the sugar in milk and water. Sprinkle yeast over top of the milk mixture and let stand at room temperature for 10 minute or until frothy. Gradually mix in remaining 2 Tbs. sugar, flour, and salt. On a lightly floured board, knead dough for about 5 minute or until smooth and elastic. Shape into a ball and place in a lightly greased bowl. Cover with a damp cloth and let rise in a warm place for 1 hour or until doubled. Meanwhile, soak mushrooms in enough warm water to cover for 30 min; drain. Cut off and discard stems; coarsely chop caps. Set aside. place wok or wide frying pan over high heat until hot. Add oil, swirling to coat sides. Add green onions, garlic, and mushrooms; stir-fry for 1 minute Stir in water, hoisin sauce, and sugar; mix well. Add cornstarch solution and cook, stirring until sauce boils down and thickens. Add pork, mix well. Remove from heat and let cool. Punch down dough, then roll on a lightly floured surface into a 12-inch cylinder. Cut cylinder crosswise into 1-inch pieces. Shape each piece into a ball; let rest for 5 minute To shape each dumpling, flatten each one ball with a rolling pin to make a 4-6-inch circle, keeping dough covered to prevent drying. Place one heaping Tbs. of filling in the center of each circle. Gather the edges of the circle over the filling; close top by pleating, pinching and twisting edges together. Place buns, pleated side down, on a baking sheet, allowing enough room for rising. Cover with a damp cloth and let rise in a warm place for about 30 minute or until light and puffy. Brush tops of buns with egg yolk. Let rise for 15 more minute Bake in a 350* oven for 18-20 minute or until golden brown.