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We are consolidating our common methods into an Abstract controller base class. Here's an example:

public abstract class CommunityControllerBase 

 /*
 Returns a select option list of Gender for use with lightning:combobox.
 See: https://help.salesforce.com/articleView?id=000212327&type=1
 */
 @AuraEnabled
 public static List<SelectOption> getGenderPicklistEntries() 
 List<SelectOption> options = new List<SelectOption>();
 Schema.DescribeFieldResult fieldResult = Contact.Gender__c.getDescribe();
 List<Schema.PicklistEntry> ple = fieldResult.getPicklistValues();
 for (Schema.PicklistEntry f: ple) 
 options.add(new SelectOption(f.getValue(), f.getLabel()));
 
 return options;
 

We are extending the class and calling it from a Lightning Component component:

public with sharing AwesomeController extends CommunityControllerBase 
 // some fancy code ...

I can use anonymous Apex to call the base class method getGenderPicklistEntries and get an appropriate result via the extending class:

AwesomeController.getGenderPicklistEntries();

However, when we call the getGenderPicklistEntries method from Lightning, we get an error:

Unable to find action 'getGenderPicklistEntries' on the controller of...

If I copy the method getGenderPicklistEntries into the extending class and comment it out on the base class, it works (finds the method and pulls the list of genders).

Why can't our Lightning component see our base method in the base class, but can see it when it's copied in the extending class?

Why can't our Lightning component see our base method in the base class, but can see it when it's copied in the extending class?

Because all @AuraEnabled methods used from JS Controller in an Aura Component need to be static. And that static methods defined in a parent class are not extended or inherited in a subclass, but they are hidden. A very good mention of this can be found on the Overriding and Hiding Methods Java docs which states:

If a subclass defines a static method with the same signature as a static method in the superclass, then the method in the subclass hides the one in the superclass.

Static methods are always associated with the Class. So if in your component you have a AwesomeController declared as a Controller, you will need to ensure that the method you are trying to invoke is available in that class.