Add class to content field (link)
This is a solution I found, but it isn't really handy to use... I really want a better solution, like something directly from twig templates.

function template_preprocess_field(&amp;$variables, $hook) {
  $element = $variables['element'];
  if ($element['#name'] == 'field_c_button_link') {
    $variables['items'][0]['content']['#options']['attributes']['class'][] = 'button';
  }
}