i don't know... depends on your level i guess.
i think i wd buy mixer for gems and wd keep the keys to learn recipes (you can find them all
here).
as you can see, all these recipes are giving 10coins income per plate, which is extremely helpful
for making money.
so far, i've learnt 7 of them, and have only 1 extra gold key waiting for cronut when i'll get to lvl40.
all bronze i'm using right when i get them: exchanging for gourmet ingreds, and making gourmet recipes as much as i can.
the only thing i don't know wht to do with tons of silver keys i have, but not so big deal since they don't take any space, so let it be.