What's the correct method to subclass a singleton class in Objective -C?

I had a similar problem, I had multiple targets that needed to have a slightly different singleton implementations: each target would include the base class + a specific subclass. This was achieved by writing the base class like so:

+ (SingletonBaseClass*) sharedInstance {
    static SingletonBaseClass * sharedInstance = nil;
    if (!sharedInstance) {
        sharedInstance = [[[self class] alloc] init];
        [sharedInstance customInit];
    }
    return sharedInstance;
}

The key difference is [self class] instead of the actual class name. That way when the we call: [SingletonSubclass sharedInstance] the correct object is instantiated.

Please note that this is a specific case, in the general case I agree with previous answers.