Pick a yeast that is kown for leaving diacetyl in the beer after fermentation is complete. There are some styles of english ales, that are known for their butterscotch notes in them.
The literature usually shows how to avoid diacetyl, but not how to produce it in a controlled manner. The problem with creating diacetyl by doing everything wrong might be the fact that you may also get other "off-flavors", which may not be desired.
Kai