Once the butter has come to room temperature and in a clean mixing bowl, cream the butter on high with the paddle attachment until lighter in color. This will take a few minutes.
Add the canned pumpkin puree and mix until just combined.
To the butter and pumpkin mixture, add in the powdered sugar, vanilla extract, and pumpkin pie spice. Use a rubber spatula to mix the ingredients together a bit before turning on the stand mixer to low to combine. You can also cover the mixer with a towel to prevent a powdery mess. Scrape the sides of the mixing bowl as needed.
Place the buttercream into two piping bags, one with the Wilton tip #5 in it and another with a Wilton star tip in it.
Decorate the cooled pumpkin sugar cookies with the buttercream, using the tip #5 bag to pipe the pumpkin shape and the star tipped bag to pipe the stem. Once all of the cookies are piped, sprinkle the cinnamon sugar mix over the cookies.
For extra soft pumpkin spice sugar cookies, place the cookies in a single layer into an airtight container. Freeze the cookies for at least an hour or up to three months. See the above section for my recommended containers. Allow the frozen cookies to defrost in the container for 30 minutes before opening the containers and serving the cookies.