Buttery or Butterscotch (Diacetyl) Aroma or Flavor
Sometimes a desireable trait, diacetyl is produced by the yeast and also by bacteria. It is present in the fermentation process and normaly decreases over time.
Do not stop the fermentation process short. Leave the beer ferment for 7 to 10 days. Higher fermentation temperatures decrease the level of diacetyl. Always watch your sanitation procedures throughout the process.
To identify exactly what diacetyl smells like, take a whiff of some imitation butter extract.