Preheat the oven to 325 degrees F and cover a baking sheet with parchment paper. Roll out the shortbread dough on a lightly floured surface into a rectangle about 1/4 inch thick. (If you have trouble with the dough sticking, try rolling it between two sheets of parchment instead.)
Cut the dough into 2x2-1/2 inch rectangles. Reroll the scraps of dough and cut more rectangles until you have 12 total.
Transfer the dough rectangles to the prepared baking sheet, and bake them for 20 minutes or until the edges are golden brown. Let them cool completely before continuing.
Divide the buttercream in half. Add a drop or two of green food coloring to one half, creating a light green color.
Add a larger amount of green food coloring to the second half so you have a bolder green color.
Set aside 1/4 cup of dark green buttercream. Fit a piping bag with a coupler and hold the bag in one hand, with the top of the bag cuffed around your hand. Place the remaining dark green buttercream in the bag, trying to keep it against one side only. Fill the other side of the bag with the light green buttercream.
Fit the piping bag with a leaf tip, a small star tip, or a grass tip. (They all work well. You can also switch tips between cookies to get a variety of grass textures.)
Pipe the frosting on top of the cookies so the top is entirely covered. Press one or two candy pumpkins into the top of each cookie. Scatter a few chocolate rocks over the cookies as well.
Transfer the remaining dark green buttercream into a piping bag fitted with a small round tip. Pipe swirls of frosting from the top of the pumpkin down to the grass to represent the vines.
Elizabeth LaBau is a cookbook author, recipe developer, and avid sugar enthusiast. She is the author of The Sweet Book of Candy Making and is the self-appointed chief sugar officer at sugarhero.com, her blog where she shares creative, modern, fun dessert recipes. She's also the candy expert at about.com, and her work has been featured in many national magazines and newspapers.