New Window class - cannot convert LRESULT Class:: to LRESULT

I have two methods in my main.cpp file that deal with creating a new window.

Firstly HWND NewWindow(...) which creates a WNDCLASSEX struct, defines some settings & registers the window.
One of the settings is

Code:

wndStructure.lpfnWndProc = WindowProcedure;
WindowProcedure in this case is the second method LRESULT CALLBACK WindowProcedure(...), this defines the events that will happen based on the Messages (I think?).

I wanted to move all of this code out of my main.cpp file into it's own class. The problem rises on the aforementioned line of code.
The method is now declared as HWND ClassName::NewWindow(...) and the error I receive is:

error C2440: 'type cast' : cannot convert from 'LRESULT (__stdcall ClassName::* )(HWND,UINT,WPARAM,LPARAM)' to 'LRESULT'
I am assuming that it is not recognising the return type as LRESULT because it is ClassName::LRESULT.

How can I get around this problem?