Use .html.twig again

source/themes/opdavies/oliverdavies/_layouts/post.html.twig

@@ -1,10 +1,10 @@
-{% extends 'default' %}
+{% extends 'default.html.twig' %}
 
 {% block body_classes 'page--blog page--blog__post' %}
 
 {% block content_wrapper %}
     <main class="col-md-9">
-        {% include 'post_header' %}
+        {% include 'post-header' %}
 
         {% block content %}{% endblock content %}