bootstrap_forms-8.x-1.x-dev/templates/bootstrap/dropbutton.html.twig

<div class="btn-group">
    <button type="button" class="btn btn-default dropdown-toggle" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
        {{ title }} <span class="caret"></span>
    </button>
    <ul class="dropdown-menu">
        {% for link in links  %}
        <li>{{ link }}</li>
        {% endfor %}
    </ul>
</div>