In a large bowl or with your stand mixer cream the powdered sugar and butter until it is fluffy. If using the stand mixer it will take about 5 minutes.
Once the sugar and butter is creamed together, add in the egg and vanilla extract.
In a medium sized bowl mix the flour, baking powder, salt and granulated sugar. Use a whisk to mix everything together thoroughly.
Slowly add the flour mixture to the sugar, butter, egg and vanilla extract mixture. Mix everything until well incorporated.
Put a handful of the freshly made dough between two sheets of parchment paper and roll until you get your desired thickness and set aside still on the parchment paper.
Repeat with the rest of the sugar cookie dough.
Put the rolled dough including the parchment paper, on a cookie sheet and refrigerate for 30 minutes.
After your dough has been rolled out and refrigerated for half an hour cut your trees with the cookie cutter from the dough.
Place the rectangles onto a sheet pan lined with parchment paper or a silicone mat, bake at 350 for 8 to 10 minutes or until the cookies have browned on the edges.
Move the cookies to a cooling rack and allow them to completely cool.
When your cookies are cooling prepare your royal icing.
Combine all ingredients in the bowl of an electric mixer fitted with the paddle attachment. Mix on low speed until smooth and creamy, about 8 to 10 minutes.
If icing is too thick, add more water. If too thin, beat icing 2 to 3 minutes more.
Next add a clear extract to make your frosting take on different flavors if you want to. This is also when you will want to color the frosting as well if you are going to color it.
Place 2/3 of your royal icing into one bowl and color it green. Divide the remainder of the icing into two bowls and color one bowl brown.
Place your green, brown, and white royal icing into their own pastry bag with number 3 straight pastry tip on each bag.
Pipe the outer border of the green on your tree. Remove the green icing from your bag and thin it out to a six-second icing and flood in the tree using a toothpick to fill in any holes.
With your brown icing add your outline to the stump. Remove the brown icing from your bag and thin it out to a six-second icing and flood in the stump using a toothpick to fill in any holes.
With your white icing add wavy lines to the tree only working on one cookie at a time. While your icing is still wet take a damp paintbrush and gently thin out the icing upward.