What ingredient is poured into a pan to make jiaozi's brown crispy lattice base?

To make jiaozi’s brown, crispy lattice base, cooks pour a flour-and-water mixture into the pan near the end of cooking.

The dumplings are first pan-fried, then the slurry is added around them and the pan is covered so the jiaozi steam. As the liquid evaporates, the flour settles and browns against the pan, joining the dumplings in a delicate, crisp sheet. When the pan is inverted onto a plate, the result is a lace-like or “ice-crystal” base.

This technique is associated with pan-fried jiaozi and resembles the crisp skirt served with some Japanese yaki-gyōza. It is not the same as simply adding oil: oil crisps the dumpling skin, while the flour-and-water slurry creates the connected lattice. Egg-based skirts are another variation, but they are not the ingredient combination asked about here.