BaseClassEditor.cs

#if UNITY_EDITOR
  [CanEditMultipleObjects]
  [CustomEditor(typeof(BaseClass), true)]
  public class BaseClassEditor : Editor
  {
    public override void OnInspectorGUI()
    {
      var t_baseTarget = typeof(BaseClass);
      var t_baseEditor = typeof(BaseClassEditor);
      
      // Always draw script reference of child first
      CustomInspectorScripts.DrawScriptReference(this); 
      
      // Draw Inspector for base
      GUILayout.BeginVertical(EditorStyles.helpBox);
      this.DrawDefaultInspector(t_baseTarget);
      /*
        CUSTOM EDITOR STUFF HERE FOR BASE CLASS
      */
      GUILayout.EndVertical();



      // If we we are being used on a child of baseclass without a custom inspector
      if (this.GetType() == t_baseEditor)
      {
        System.Type t_type = target.GetType();
        var target_types = new List<System.Type>();
        while (t_type != t_baseTarget)
        {
          target_types.Add(t_type);
          t_type = t_type.BaseType;
        }
        target_types.Reverse();
        foreach (var type in target_types)
          this.DrawDefaultInspector(type, false);
      }
    }
  }
#endif

ChildClassEditor.cs

#if UNITY_EDITOR
[CustomEditor(typeof(ChildClass), true)]
public class ChildClassEditor : Button_AEditor
{
  public override void OnInspectorGUI()
  {
    var t_childTarget = typeof(ChildClass);
  
    base.OnInspectorGUI();
    this.DrawDefaultInspector(t_childTarget);

    /*
      CUSTOM EDITOR STUFF HERE FOR CHILDCLASS
    */
  }
}
#endif