Use a double boiler (a heatproof bowl set on top of a saucepan of simmering water), to melt the chocolate and shortening, stirring until smooth. Remove from the heat.
Pour the melted chocolate and shortening onto the bottom side of a 9x13 glass baking dish. Use an offset spatula to spread the chocolate out into a thin layer, near the edges.
Set the pan in the refrigerator and chill for several minutes, just until the chocolate is firm, and you don't see any wet patches in the center.
Remove the pan from the refrigerator. Using a putty knife or bench scraper, hold it at a 45 degree angle and push the chocolate away from you, letting it curl up. You can make the curls as wide or as narrow as you like; I made mine about 1/4-1/2 inch wide.
If the chocolate flakes instead of curling, then it's too cold; wait 20-30 seconds and try again. If the chocolate doesn't curly, and just pushes up in sheets, then it's too warm. Place the pan back in the refrigerator for 45-60 seconds, and try again.
You will need to work quickly, but be patient, You'll likely need to return the pan to the refrigerator several times throughout the process.
As you make your curls, use tweezers to transfer them to a container. Cover and refrigerate until needed, then use tweezers to place them on your cake (the heat of your fingers can quickly melt the curls).