GUI button callback sample for Unity5

ButtonCallback.cs

using UnityEngine;
using System.Collections;
using UnityEngine.UI;

public class ButtonCallback : MonoBehaviour {

	// Use this for initialization
	void Start () {
        Button button = this.GetComponent<Button>();
        button.onClick.AddListener(onClicked);
    }
	
	// Update is called once per frame
	void Update () {
	
	}

    protected virtual void onClicked()
    {
        Debug.Log("onClicked");
    }

}