What grammatical name is given to this expression 'whose cake is the best'

It's Adjectival clause..

The grammatical name for whose cake is the best is adjectival clause

WHAT IS IT FUNCTION ?

adjectival

whose cake is the best

Adjectival clause

ADJECTIVAL CLAUSE

If you use correct capitalization and punctuation, it's called a question ... otherwise known as an interrogative sentence.

Noun clause

YES