Is butter a mass noun

Yes, the noun 'bread' is a mass noun (also called an uncountable noun), a Word for a substance. A partitive noun (also called a noun counter) is a noun used to count or quantify a mass noun, for example a 'loaf' of bread or a 'slice' of bread.

The plural noun 'breads' is a Word for types of or kinds of bread, for example 'a selection of fresh baked breads'.