"Read more" button for Sparkling WordPress theme

Is there any way so that the “read more” button ONLY appears on posts that have the “read more” truncate option?

Just went through this myself.

Remove this from content.php

<p><a class="btn btn-default read-more" href="<?php the_permalink(); ?>" title="<?php the_title_attribute(); ?>"><?php _e( 'Read More', 'sparkling' ); ?></a></p>

Add this to functions.php

add_filter( 'the_content_more_link', 'modify_read_more_link' );
function modify_read_more_link() {
return '<a class="btn btn-default read-more" href="' . get_permalink() . '">Read More</a>';

Solution provided by the theme author with modification by @kalliste here on the support forum

This worked splendidly. MANY MANY thanks!