1. Sift flour, baking powder, and salt into a bowl. Put butter and sugar into the bowl of an electric mixer fitted with the paddle attachment. Mix on medium-high speed until pale and fluffy. Mix in egg and vanilla. Reduce speed to low; gradually mix in flour mixture. Wrap in plastic; refrigerate 45 minutes.

2. Preheat oven to 325 degrees. Transfer dough to a lightly floured work surface. Roll out to a generous 1/8 inch thick. Using a tree-shape cookie cutter, cut out cookies, and space 2 inches apart on parchment-lined baking sheets. Reroll scraps, and cut out. Refrigerate until firm, about 15 minutes.

3. Bake until golden, 12 to 15 minutes. Let cool completely on sheets on wire racks. Drizzle cookies with icing; sprinkle with nonpareils. Let set.