Learn Russian: Вы замечательно готовите!

You cannot use что за directly with a verb like готовите, because что за must be followed by a noun in the Nominative case. Instead, you could change the phrasing to compliment the cook: Что за замечательный повар! (What a wonderful cook!).

I remember learning варить for 'to cook'. Why is готовите used here instead?

Варить specifically means 'to boil' or 'to cook in liquid' (like soup or pasta). Готовить is the broad, general verb for 'to prepare food' or 'to cook' in general. When complimenting someone's overall culinary skills, готовить is the natural choice.

Why is there no л in готовите? I thought the stem was готовл- based on я готовлю.

For second conjugation verbs ending in -вить, the consonant mutation that adds an л only applies to the first-person singular (the я form). All other present tense forms, including вы готовите and он готовит, drop the л and use the regular в stem.

Could I use the adjective замечательная here if I am thinking about the food (еда)?