Potato in the dough is what makes a pierogi different from any other dumpling — it goes soft and slightly chewy rather than crisp, and it takes filling without splitting. Chef Emily was clear that the potato has to be properly dry before it goes in, so riced or mashed without milk, or the dough turns claggy. As organizer I'd flag that people need a potato ricer or at least a masher, which two of ours didn't have.
Jade