Preheat the oven to 325 F and cover a baking sheet with parchment paper. In a bowl, whisk together the flour, sugar, pumpkin pie spice, and salt.
Add the cubed butter to the flour, and use a pastry blender or two knives to cut the butter into the shortbread, working it in until it looks like coarse crumbs. Finish mixing the dough by hand, kneading the dough gently, just until it smooths out.
Roll the shortbread dough out 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 squares 2 inches by 2.5 inches. Re-roll the scraps of dough and cut more squares until you have 12 squares of dough. Transfer them to the prepared baking sheet and bake them for 18-20 minutes, until the edges are golden brown. Cool them completely before continuing.
To Make the Buttercream:
Place the butter, vanilla, sugar, and milk in the bowl of a large stand mixer fitted with a paddle attachment. Mix on low speed until the sugar is moistened. Scrape down the sides of the bowl, then beat again on medium speed until light and fluffy, 2-3 minutes. You can add more milk or sugar, if desired, to get the consistency you want.
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 that you have a bold green color.
Set aside 1/4 cup of dark green buttercream. Fit a piping bag with a coupler, and hold it 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 the cookies. Scatter a few chocolate rocks over the cookie 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 stems.