01 - Preheat oven to 425°F. Line a baking sheet with parchment paper.
02 - Heat a skillet over medium heat. Add diced Chinese sausage and cook for 2-3 minutes until fragrant and lightly browned. Drain excess fat and set aside to cool.
03 - In a large mixing bowl, whisk together flour, baking powder, baking soda, salt, and sugar until thoroughly combined.
04 - Add cold butter cubes to the flour mixture. Use a pastry cutter or your fingertips to work the butter into the flour until the mixture resembles coarse crumbs with some pea-sized pieces remaining.
05 - Stir the cooled sausage, scallions, and sesame seeds (if using) into the flour-butter mixture until evenly distributed.
06 - Pour cold milk into the bowl. Mix gently with a spatula or wooden spoon just until a shaggy dough comes together. Do not overmix.
07 - Turn dough onto a lightly floured surface. Pat into a 1-inch thick rectangle. Fold dough in half, pat out to 1-inch thickness again, and repeat folding once more to create flaky layers.
08 - Use a 2.5-inch round biscuit cutter to cut out rounds. Press straight down without twisting. Gather scraps, gently pat together, and cut additional biscuits. Handle dough as little as possible.
09 - Place biscuits on prepared baking sheet. Arrange close together for softer sides, or space 2 inches apart for crispier edges.
10 - Brush tops of biscuits with beaten egg for a glossy golden finish, if desired.
11 - Bake for 16-18 minutes until tops are golden brown and biscuits are cooked through.
12 - Let biscuits cool on the baking sheet for 5 minutes before transferring to a wire rack. Serve warm.