Preheat the oven to 350° F and line a 9x13 baking dish with parchment paper with an overhang for easy removal.

Combine the softened unsalted butter and cream cheese in a large bowl.

Beat the granulated sugar until light and fluffy.

Beat in the egg and the vanilla extract.

In a separate bowl, combine the spoon and leveled all-purpose flour and the salt.

Slowly beat the dry ingredients into the wet ingredients until a soft dough forms.

Place the cookie dough inside the lined baking dish and spread evenly across the dish. (To make this easier, place parchment paper on top of the dough and use a glass to spread the dough from the center of the dish to the edges.

Make sure that the dough is evenly spread inside the dish before baking.

Bake for 35-40 minutes. The edges should be a golden brown color and the center set.

Check the center of the dish with a toothpick and if desired, bake for a few minutes extra at a time if you want firmer sugar cookie bars.

Remove from the oven and allow to cool for 10 minutes before lifting the bars out of the dish and transferring them to a wire rack to cool completely.

Once completely cooled, slice the bars into 12 bars if decorating with Christmas trees.

Starting with room temperature butter, cream the butter for 7 to 8 minutes.  

Slowly beat the sweetened condensed milk into the butter until well incorporated.

Stir in the vanilla extract and slowly beat in the powdered sugar.

Beat until stiff peaks form when the beater is pulled away from the frosting. If needed, add more powdered sugar a little at a time.

Divide the frosting into separate bowls and add food coloring to one bowl.

Fill a piping bag fitted with a small piping tip and decorate each bar as desired. 

If making Christmas trees, sprinkle with sanding sugar and add a star sprinkle on top of the tree. 

Frost around the tree and decorate with sanding sugar. 

Let the frosting harden before storing or stacking to prevent the icing from smearing.