How to make a button?

The “(more…)” link gets filtered through the_content_more_link in get_the_content(). You can hook into this function and let it return whatever you want.

add_filter('the_content_more_link', 'more_button');
function more_button($more_link) {
    return '<button class="readmorebtn" onclick="' .
        esc_attr('window.location="' . get_permalink() . '"') .
        '">Read more</button>';
}