Preheat the oven to 350°F/180°C and prepare a large cookie sheet with parchment paper.
In a medium mixing bowl, combine the dry ingredients for the sugar cookies. Mix together the flour, baking soda, baking powder, and salt. Mix well and set aside.
In a separate mixing bowl, beat together the butter and granulated sugar until smooth.
Mix in the egg and vanilla and beat until fully combined.
Add in the dry ingredients and mix until fully combined and the cookie dough is smooth.
Cover and place dough inside the refrigerator for 30 minutes.
When the dough has properly chilled, you can begin scooping. You will need to scoop three sizes of cookie dough for your Christmas trees. Scoop a 1, 2, and 3 teaspoon ball onto each pan, repeating four times for a total of 12 cookies on each pan, in three different sizes.
Bake the cookies for 6-7 minutes. When done, remove from the oven and allow to cool on a cooling rack until they reach room temperature.
Once the cookies are at room temperature, begin to prepare your frosting. In a large mixing bowl, combine butter and powdered sugar until smooth.
Add in the vanilla extract, salt and milk and continue to mix until fully combined.
Mix in the green food coloring until you have reached your chosen shade of Christmas tree green.
Using a rubber spatula, scrape the edges of the bowl, gathering all of the frosting and spoon into a pipe bag with a star tip attachment.
Starting with the largest cookie size, pipe the frosting over the cookie, covering it completely in one layer.
Add the medium sized cookie on top and pipe frosting over, covering it completely.
Add the smallest cookie size on top of the frosting and finish the tree off with a small upwards piping, creating the tree tip.
Add Christmas sprinkles on the exposed frosting edges for the tree “ornaments” and one yellow star on top. Serve as is or allow to chill overnight for frosting to harden.
Notes
Keep the Dough Chilled: You'll be baking multiple trays of cookies, so be sure to put the dough back in the fridge while the first batches are baking. If the dough is too warm, the cookies will spread too much.
Stay Organized: I found it was easiest for me to portion out the dough as I show here, making all three sizes and enough for four trees on each tray of cookies. You may prefer to do a pan of all 1-inch cookies, and separate pans for the other sizes. Either way, keep track so that in the end you have the right amount of cookies in each size to create the trees.
For Smooth, Round Cookies: Use a round cookie cutter that is larger than the cookies to gently swirl them in the pan to smooth the edges while the cookies are still warm.
Make 1 Tree at a Time: It might sound like a good idea to make your sugar cookie Christmas trees in an assembly line fashion, but it isn't. If the frosting dries, you will not be able to stick the additional cookies to it, and the sprinkles won't stick either.
These fit perfectly into cupcake liners, so feel free to use them if you want to include stacked Christmas tree cookies on a platter and they need to be laid on their sides.