With an electric mixer, cream butter, salt, sugar and vanilla seeds until light.
2
Beat in egg white until well incorporated, scraping sides and bottom of mixing bowl to insure that all white has been incorporated.
3
Beat in extract and milk until well incorporated.
5
Fit a piping bag with a large, plain pastry tip (I use Ateco #806, but slightly larger or smaller sizes will also work). Fill bag with about 1/3 of the batter. Holding the filled bag perpendicular to a parchment-lined baking sheet, pipe batter into nickel-sized mounds, about 1 inch apart. To make neat, well-shaped cookies, as you finish piping each round, sweep the pastry tip horizontally off to the side with a slight curving motion. (The cookies should cover 2 to 3 standard baking sheets. Batter will hold for a while after it is piped, so trays can be baked one or two at a time, as your oven allows.)
6
Bake at 350°F for about 15-20 minutes, until cookies are lightly browned, rotating trays every 5 minutes or so for even coloring. Cool completely before storing. Cookies will keep in an airtight container for at least a week.
