Beat the butter and sugar on medium speed until light and fluffy, about 3 minutes.

Mix in the egg and vanilla, then add the combined flour, baking powder, and salt. Beat on low just until a dough forms.

Roll the dough ¼ inch thick between parchment sheets, chill for 1 hour, then cut sweater shapes with a floured cutter.

Bake at 350°F for 9 to 11 minutes until the edges are pale golden. Cool on the pan for 3 minutes, then transfer to a rack.

Whip the powdered sugar, meringue powder, and 6 tablespoons water into a smooth icing. Add water as needed until it flows and ribbons back in 10 seconds. Tint with gel coloring.

Outline and flood the cookies, then add decorative details. Let the icing dry for 2 to 3 hours before serving.