How do call the template for a module block
The markup render #type takes a string for its #markup element property, not a render array. However you do not need to wrap your custom theme/template with the markup type.

  public function build() {
    return array(
      '#theme' =&gt; 'scadfashblock',
      '#source_text' =&gt; $this-&gt;siteContent(),
    );
  }

  public function siteContent() {
    return some_function_returning_html_from_user();
  }


Then implement scadfashblock in hook_theme, provide a template_preprocess_scadfashblock and a templates/scadfashblock.html.twig with your markup.