Who laid the foundation cornerstone of the Capitol building?

The symbolic cornerstone was laid on Wednesday, September 18, 1793, during the first large public event staged in the federal city. Contemporary Masonic practice included the laying of an inscribed metal plate along with a cornerstone. The plate was handed to George Washington, who stepped down into the foundation trench, laid the plate on the ground, and lowered the cornerstone onto it.

Source: Architect of the Capitol