<?php
defined('C5_EXECUTE') or die("Access Denied.");

class ContentOfferteBlockController extends BlockController {
	...

	public function view(){
		//rendering stuff
	}

	public function action_add_to_favourites (){
		$bt = BlockType::getByHandle('content_offerte');
		$bt->render('view');

		exit;
	}
}

//generates this error:
Fatal error: Call to a member function getProxyBlock() on a non-object in /core/libraries/block_view.php on line 39