Lately I have heard many people using what in place of which in adjectival phrases:
- This cake, what is my favorite, makes me happy.
- This cake, which is my favorite, makes me happy.
Is the first usage grammatical? Which is the preferred usage?
To clarify we're speaking of two different constructs using which or what as part of an adjectival phrase:
This cake, which is my favorite, is very fattening.
This cake, what is my favorite, is very fattening.
Which is clearly the better usage in this scenario.
I've certainly seen what used in this way (as a replacement for which), but I believe it to be a colloquial usage. Merriam-Webster calls this usage chiefly dialect.
I believe this usage to be chiefly in dialects of BrE (and perhaps Southern AmE).