In a medium bowl, whisk together the flour, baking powder, and salt.

In a larger bowl, cream together the softened butter and granulated sugar until light and fluffy, about 5 minutes.

Add the egg, followed by the vanilla extract and milk until combined.

Gradually stir in dry ingredients until a cohesive dough forms.

Divide the dough into two equal portions, wrap in plastic wrap, and refrigerate for at least an hour or overnight.

Preheat your oven to 350°F (175°C) and line baking sheets with parchment paper.

On a lightly floured surface, roll out one portion of the chilled dough to about 1/4-inch thickness.

Use a Christmas tree-shaped cookie cutter to cut out cookies and place them on the prepared baking sheets.

Bake for 8-10 minutes or until the edges turn slightly golden.

Allow cookies to cool completely on a wire rack.

In a clean bowl, whisk together the powdered sugar, egg white (or meringue powder), lemon juice, and enough water to reach a thick consistency.

Divide the icing into separate bowls for coloring, stirring green food coloring into one bowl.

Cover each cookie with the green royal icing to form the base.

Allow icing to set for about 10-15 minutes before adding additional details.

Use other colors to create decorations like tinsel and ornaments.

Sprinkle with red and white nonpareils and optional star-shaped sprinkles.

Let decorated cookies sit until the icing fully sets, taking approximately 1-2 hours.