With an electric mixer, cream butter, salt, sugar and vanilla seeds until light.

Beat in egg white until well incorporated, scraping sides and bottom of mixing bowl to insure that all white has been incorporated.

Beat in extract and milk until well incorporated.

Whisk together flour and baking powder and add to butter mixture. Mix just to incorporate, scraping down sides and bottom of mixing bowl to insure that all flour has been incorporated.

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.)

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.