Paul,  ‘ (le) reste ’ is a noun, not an adjective and is always masculine. 

Nouns don’t take agreement with each other in phrases or expressions. 

 https://www.wordreference.com/fren/reste

Bonjour Paul,

La ganache is a pâte de chocolate et crème.

During the preparation of the dish, not all of la ganache was used up.

Later on, at the completion;  the remainder (le reste) was required to be applied.

Hope this helps.

Bonne journée

Jim