Are there any single-pilot aircraft certified in the Part 23 Commuter category?
It looks like it's possible but not common. There's an STC for the Twin Otter that authorizes single-pilot operations in the commuter category if various upgrades have been made. At least, that's my reading of it:


  The Twin Otter DHC-6-300HG™ type rating designation is DHC-6HG and may
  be operated with or without a second in command (SIC) with certain
  limitations. The Airplane Flight Manual Supplement (AFMS) lists
  equipment that must be installed and operative to operate the aircraft
  single pilot.


And:


  FAA STC SA02682LA incorporates upgraded engines, upgraded propellers,
  increased maximum takeoff weights (MTOW), structural and aerodynamic
  modifications, and the aircraft category and design characteristics
  were updated to Title 14 of the Code of Federal Regulations (14 CFR)
  part 23 commuter category.