For The Sugar Cookie Bars:

1. Heat oven to 350 degrees. Lightly coat the pan in cooking spray or with a thin layer of butter. Line the pan with 2 perpendicular sheets of parchment paper. The paper should extend past the rim of the baking pan. Place baking pan in the freezer (this helps the dough spread more easily).
2. In a medium mixing bowl, whisk together the flour and salt. Set aside.
3. In the bowl of a stand mixer  fitted with the paddle attachment, beat the butter and cream cheese at medium speed until fully incorporated, about 1 minute. Reserve the butter wrapper.
4. Add the sugar and beat until smooth, about 1 minute. 
5. Add the egg and vanilla and beat on low speed until well mixed, about 1 minute. Turn off the mixer, scrape the edges of the bowl. 
6. Add the flour mixture, mixing on low speed until just blended, about 1 minute.
7. Using a damp spatula, scrape the dough into the prepared baking pan. Use damp hands to press it into an even layer. To ensure that the dough is as flat as possible, use the greased side of the reserved butter wrapper to press the dough into the pan, paying particular attention to the corners.
8. Bake for 20-25 minutes or until the edges edges turn golden brown and an inserted toothpick comes out with a few moist crumbs. Set pan on a wire rack to cool completely before slicing. 
9. Slice the cooled bars into 2 even  halves.  Use a sharp knife to slice each half of  the bars into 4 triangles. You will have 8 in total. See tutorial in the post for step-by-step pictures.

To Decorate:

1. Prepare your green buttercream icing and load it into a piping bag fitted with a 1M piping tip. 
2. Use the piping tip to apply small starts that cover the surface of the sugar cookie bar. Start at the bottom and work your way to the tip of the tree.
3. Strategically add sprinkles (ornaments) to your Christmas tree.
4. Unwrap a Tootsie Roll and mold it into the shape of a tree trunk. Use a butter knife to add lines for the "tree bark." Use a small amount of icing to secure the "trunk" to the "tree."