functions.php

<?php

/**
 * Back to Portfolio Link
 * @author Bill Erickson
 * @link http://www.billerickson.net/genesis-portfolio/#comment-670383
 */
function be_back_to_portfolio_link() {

  // Only run on single portfolio items
  if( ! is_singular( 'portfolio' ) )
    return;
    
  echo '<p><a href="' . get_post_type_archive_link( 'portfolio' ) . '">Back to Portfolio</a></p>';
}
add_action( 'genesis_entry_content', 'be_back_to_portfolio_link', 20 );