Render block via twig template
I was missing the implementation of hook_theme. In order to use the template you need to declare it there.
FX

/**
 * Implements hook_theme().
*/
function mymodule_theme() {
  return array(
    'block__newsletter' =&gt; array(
      'render element' =&gt; 'elements',
    ),
  );
}


You should then have a templates/block--newsletter.html.twig.