1. Heat oven to 350 degree F. Combine flour, baking powder, and salt in a bowl. Scrape out seeds from vanilla bean. Beat butter, sugar, and vanilla seeds (or vanilla extract if using) in a large mixing bowl until light and fluffy. Beat in egg. At low speed, beat in flour mixture until blended. Spoon dough into a large pastry bag fitted with a 1/2-inch star tip. Pipe dough into 2-1/2-inch-wide circles 1 inch apart on 2 ungreased cookie sheets. Bake until cookies are golden brown at edges, 10 to 12 minutes.
2. Meanwhile, in a large bowl, whisk confectioners sugar and enough hot water to make a thin glaze. Transfer cookies to a wire rack set over a piece of waxed paper. Immediately brush hot cookies with glaze. Let stand 30 minutes until glaze sets. Makes about 2 dozen cookies.