my_view.rb

class MyView < Mustache
  def markdown(str = nil)
    if str
      Markdown.new(str.to_s).to_html
    else
      lambda { |text| markdown(render(text)) }
    end
  end
end

template.mustache

<div class="column main">
  {{# markdown}}
    {{{ description }}}
  {{/ markdown}}
</div>