DLSubclassAwareSingleton is the result of a random brain fart turned into a basic yet usable proof of concept:

An Objective-C category on NSObject implementing the common singleton pattern with the addition of allowing subclasses to provide their own shared instance without any additional code.

Word of caution: Being a common code spell think twice before using a singleton.

Anyway. A code snippet probably says more than a thousand words:

#import <Foundation/Foundation.h>

#import "DLFoo.h"
#import "DLFooBar.h"

int main(int argc, const char * argv[]) {

    @autoreleasepool {

        // DLFooBar inherits from DLFoo and doesn't
        // implement any singleton logic of its own.

        // Next line returns instance of class DLFoo:
        DLFoo *foo = [DLFoo sharedFoo];

        // Next line returns instance of class DLFooBar:
        DLFooBar *foobar = [DLFooBar sharedFoo]; 

    }
    return 0;
}