1. Make White Dough — In a medium bowl, whisk together flour and salt. Add boiling water and mix until a shaggy dough forms. Cover and let rest for 15 minutes.
4. Knead the Dough - After the dough has rested, knead each dough for 2 to 3 minutes or until smooth. Cover and allow to rest for an additional 30 minutes.
5. Make Filling - Meanwhile, heat oil in a large skillet over medium heat, and cook scallion whites, cabbage,shiitake mushrooms, and minced garlic until tender. Transfer to a large bowl. Once cooled, add ground pork, scallion greens, soy sauce, oyster sauce, sugar, grated ginger, and white pepper. Mix until incorporated. Cover and set in the fridge until needed.
6. Form Dumpling Wrappers - Divide the white dough into 2 equal pieces and roll each one into a 7-inch log. Next, roll the green dough roughly into a 7x4 inch rectangle. Then wrap the green dough around 1 white log, and with the seams pinched together, roll into a long 9-inch log. Repeat with the purple dough and place both logs in the fridge for 20 minutes. Once firm, slice each log into 12 equal disks and lightly dust both sides with flour. Using the palm of your hand, flatten each disc and then roll into a flat circle, about 3.5 to 4 inches in diameter.
7. Assemble Dumplings - Fill dumpling wrapper with a heaping tablespoon of filling and fold into a halfmoon. Pinch the top of the dumpling to seal it shut. Then tuck the corner edges inward and seal the seams all around. Squeeze both sides of the dumpling gently with your thumbs upward, 5 to 6 times to form leaves.8. Steam Dumplings - Line a bamboo steamer basket with parchment paper and cook dumplings for 1o to 12 minutes or until the internal temperature reaches 160ºF. Enjoy!