To unlock your cooking skill post 225, you have to complete a quest out of Tanaris. Here are the steps you need to take:
1. Go to the cooking trainer in the The Drag in Orgrimmar. She will give you a quest to find Dirge Quikcleave in Tanaris (he is in Gadgetzan). Note: you have to be level 40 and have your skill at 225 to get the quest.
Go to Gadgetzan and see Dirge Quickcleave. He will give you a quest called "Clamlette Surprise." For this quest, you will have to collect 10 Zesty Clam Meat (drops off the Nagas in many zones - actually, they will drop Big-Mouth Clams, but when you open them, the meat inside is what you need), 20 Alterac Swiss (this is actually purchased - I got mine from the Innkeeper in Hammerfall, but I have also seen them on the Innkeeper in Shadowprey), and, lastly, 12 Giant Eggs. The eggs are the hardest part. They drop off the Rocs in Tanaris, but are semi-rare. It took me several hours to get the eggs. You can also get the eggs from the Wildkin in Hinterlands.
After you collect the ingredients, you turn them back into Dirge and you get XP and 20 Clamlette Surprise (a nice bonus stat food). You will then have the ability to train to 300 in cooking.
However, there have been reports that some people have completed the quest and not been able to unlock the ceiling on cooking, so it stays at 225. There has been some theory that you *must* go to Orgrimmar and get the starter quest in order to make things work properly.
Good luck!