Whisk together flour, baking powder, and salt. Beat butter and sugar until light and fluffy, about 2 minutes. Add egg and vanilla, mix until combined, then gradually beat in dry ingredients. Shape dough into a disc, wrap, and chill at least 1 hour.

On a floured surface, roll dough to ¼ inch thick. Cut stocking shapes with a cutter or a knife and template. Place on parchment-lined baking sheets.

Preheat oven to 350°F (175°C). Bake 8 to 10 minutes until edges are pale golden. Cool on sheet 2 minutes, then transfer to a wire rack to cool completely.

Whisk powdered sugar, 2 tbsp milk, corn syrup, and vanilla until smooth. Add milk ½ tsp at a time to reach a slow-flowing consistency. Divide and tint with gel color.

Glaze cooled cookies and decorate with sanding sugar or nonpareils while wet. Let set 20 minutes before serving.