Adding custom url to readmore link using get_permalink()

I suspect the issue is that $recipe_url is undefined in the portion you’re trying to echo it. Can you post a link to the full code of recipe_options.php? Use pastebin or something.

Try:

href="https://wordpress.stackexchange.com/questions/114230/<?php 
        $recipe_detail_xml = get_post_meta($post->ID,"recipe_detail_xml', true);
        $cp_recipe_xml = new DOMDocument ();
        $cp_recipe_xml->loadXML ( $recipe_detail_xml );
        $recipe_url = find_xml_value($cp_recipe_xml->documentElement,'recipe_url');
 echo $recipe_url ;
?>"