Split GUI and Core code (Architecture)

[BlackMamba]

Hello,
I have developped an application with a GUI and some applicative code at the same time.
Most of the time, there is a GUI class with a pointer to the core class for example
@
class GUI_Animal{
private:
Animal* animal;
}
@

Now, I would like to split the GUI from the core code and make a dll from the core to give an API to users and also give the possibility for other people to make their own GUI.
What would be the best solution for that? Add an interface class?

Cheers