Call the author partial rather than simply placing the author in the content.

layouts/posts/single.html

@@ -17,7 +17,7 @@
             <strong>{{ .Params.subtitle | markdownify }}</strong>
             {{ end }}
             {{ if .Params.author }}
-            <pre>by {{ .Params.author }}</pre>
+            <pre>{{ partial "author.html" . }}</pre>
             {{ end }}
 
             {{ if .Params.Cover }}