Beat the softened butter and granulated sugar for 2–3 minutes, until light and creamy. Beat in the egg and vanilla extract.

Whisk the flour, baking powder, and salt in a separate bowl. Gradually add the dry ingredients to the butter mixture and mix just until a soft dough forms.

Divide the dough into two portions, flatten each portion into a disk, wrap, and refrigerate for at least 30 minutes. Chilling for 1 hour makes rolling especially easy.

Heat the oven to 350°F (175°C) and line baking sheets with parchment paper. Roll the dough on a lightly floured surface to about 1/4 inch (6 mm) thick.

Cut out Christmas tree shapes and transfer them to the prepared baking sheets, leaving about 1 inch between cookies. If the dough softens, briefly chill the cut shapes before baking.

Bake for 10–12 minutes, until the edges are set and just beginning to turn pale golden. Cool the cookies on the baking sheet for 5 minutes, then move them to a rack to cool completely.

Beat the sifted confectioners’ sugar, meringue powder, vanilla, and 5 tablespoons of warm water until smooth. Add the remaining water a few drops at a time until the icing is pipeable or spreadable, then tint it green.

Decorate the completely cooled cookies with royal icing and add sprinkles, sanding sugar, nonpareils, mini candies, or edible stars while the icing is wet. Let the icing set for several hours before stacking or packaging.