TwigMailGenerator + embed tag

some_mail.html.twig

{% block subject 'Hello World' %}

{% block body_html %}
{% embed "layout_mail.html.twig" %}
    {% block content %}
        I'm the <b>content</b>.
    {% endblock %}
{% endembed %}
{% endblock %}

{% block body_txt %}
{% embed "layout_mail.txt.twig" %}
    {% block content %}
        I'm the **content**.
    {% endblock %}
{% endembed %}
{% endblock %}