In a large bowl, mix your sugar, butter, eggs, flour, and baking powder until a smooth dough forms. Chill the mixture in the refrigerator for at least 1 hour for easier handling.

Roll out the chilled dough on a floured surface to about ¼ inch thick. Use cookie cutters to create tree shapes, ensuring you have enough pieces for stacking.

Preheat your oven to 350°F (175°C). Arrange the shaped cookies on lined baking sheets and bake for 8-10 minutes, or until the edges are lightly golden.

Once baked, remove the cookies from the oven and let them cool on wire racks completely before decorating.

In a bowl, combine powdered sugar, egg whites (or meringue powder), and a little water. Mix until you reach a thick consistency suitable for piping.

Carefully stack the cooled cookies with a generous layer of royal icing between each cookie to hold them together securely.

Use the prepared icing to enhance your cookie trees. Swirl, pipe, and sprinkle to create festive designs and colors for your trees.

Allow the decorated trees to set at room temperature for at least 2 hours, ensuring the icing is completely hardened before serving or gifting.