。
What a wonderful cook she is!
Literal
She [topic-は] cooking [subject-が] how skilled [wonder-だろう].
なんと~だろう is an exclamatory pattern meaning 'how X!' or 'what a X!' — なんと expresses surprise or admiration at the degree of something. Combined with 上手, it creates 'how skilled she is!' The のだろう adds a reflective, wondering tone.