layouts/_default/_markup/render-heading.html (5 lines of code) (raw):
<h{{ .Level }} id="{{ .Anchor | safeURL }}">{{ .Text | safeHTML }}
{{- if and (ge .Level 1) (le .Level 4) }}{{" " -}}
<a class="headerlink" title="Permalink" href="#{{ .Anchor | safeURL }}">¶</a>
{{- end -}}
</h{{ .Level }}>