I start by gently heating heavy cream and pouring it over the chocolate chips. Stir until smooth and glossy. Then, let the filling cool slightly so it thickens but remains pourable.
Warm the milk and mix it with sugar and yeast to activate. Combine with flour, wheat starch, egg white, and shortening. Knead until soft and elastic. Cover and let it rise until doubled in size, about 1–2 hours.
Divide dough into small portions and flatten each into a circle. Place a spoonful of chocolate filling in the center. Gather edges and pinch tightly to seal, then place each bun on a small parchment square.
Bring water to a simmer in your steamer. Steam buns for about 12–15 minutes, then let them sit in the steamer for 5 minutes to prevent collapse. Serve warm for the best gooey chocolate effect.