I used a plastic sandwich bag with a tiny bit of the corner cut off, as a piping bag to drizzle the chocolate over the bars. (veganmotherhubbard.net)
Tip: Or snip a very small bit of the corner off of a Ziploc bag. (carrotsandspice.com)
• Tip: Or snip a very small bit of the corner off of a parchment cone or Ziploc bag • Hold the piping bag at a 45 degree angle above the cookie where you want to start the outline. (thedaringkitchen.com)