Combine soy sauce, brown sugar, water, fennel, ginger, clove, cardamom, bay leaves, cinnamon, and star anise in a small pot. Bring to a gentle simmer and cook until the sugar dissolves and the sauce thickens slightly. Strain out the solids and let cool. Store leftovers for braises.

Stir flour and salt together, then add water gradually and mix into a stiff, shaggy dough. Knead until smooth, cover, and rest 30 minutes so the gluten relaxes. The dough should be firm but pliable.

Roll the dough into a long sheet about ⅛-inch thick, dusting with flour to prevent sticking. Cut into ½-inch wide strips, then lightly dust and gently stretch each strip to about pencil-width, keeping them separate.

Bring a large pot of water to a rolling boil. Drop in the noodles and cook until just cooked through, about 4–5 minutes for thick ribbons. Rinse briefly in cool water to stop cooking, then drain well.

In each bowl, whisk 1 tbsp sweet spiced soy sauce, 1 tbsp chili oil, and 2–3 tsp thinned sesame paste. Add ½ tsp roasted soybean powder, ½ tsp crushed garlic, and pinches of Sichuan peppercorn powder and mushroom seasoning. Toss in the cooled noodles, coat thoroughly, and serve at once, slightly chilled.