In a bowl with an electric mixer cream the butter with 1/2 cup of the confectioners' sugar and beat the mixture until it is light and fluffy. Beat in the zest and the vanilla, add the poppy seeds, the flour, and the salt, and beat the mixture until it is just combined. Press the mixture evenly into a buttered 8-inch square baking pan and bake it in the middle of a preheated 300 degree oven for 30 to 35 minutes, or until it is pale golden. Let the shortbread cool in the pan on a rack for 10 minutes, cut it into 24 bars, and let the bars cool completely in the pan. Sprinkle the bars with the additional confectioners' sugar.