mass and variable nouns

Hi,

I was told that a mass noun can be used as uncountable when you use to refer to it as a substance and a variable as uncountable when refer to it generally.

When you refer something like meat, butter and shampoo as a substance, does a person using it as such use just like a real piece meat, packet of butter and bottle of shampoo with their natural shapes, smell, and feel to them? Or is he using the likes of the variable noun 'pizza' generally, knowing using them as such entails using them without natural physical attributes that make up such noun as 'pizza' you see when you are putting your fork and knife in it to taste a piece of it.

I am asking because I have a hard time choosing which version to use, countable or uncountable, especially for such nouns known as 'mass' nouns. For a variable noun like 'pizza' or 'tomato', I am sure you can say, "my favorite food is pizza or my favorite fruit is tomato". Right?

For 'mass' nouns:

My favorite source of protein is meat.

My favorite thing to put on a piece of bread (Is 'bread' OK?) is butter.

Are these correct?