"Are" would be the most correct way but people would still know what you said if you put "is"

I am on the fence. Sometimes food is countable and in your case may not be. But lets say for arguments sake, it were countable... Because you will name more than one item, you would simply add an s to food. Or change the word to - favorite 'meal' (which can have multiple items in one set) "My favorite meal is egg and milk. "is" because the meal is one thing. But foods makes them into different things. "My favorite foods ARE egg and milk. (or eggs and milk.) eggs if you think of then as different eggs. Egg if you think about them cooked into ONE pile of egg. I hope you followed all that. So many picky rules that can do different things if you can just straiten them out!

This example does not illustrate well the point you are trying to make. It would be better in this example to say "my favorite foods are eggs and milk". To change the wording, you could say "my favorite meal is eggs and milk". That would be correct.

food + is

It's okay. Food is uncountable. My favourite food is eggs and milk. Eggs and milk are my favourite food.