Beat together the butter, sugar and salt until light and fluffy. Add in egg white, vanilla and milk. Slowly beat to incorporate the flour and baking powder, mixing until totally combined.
Refrigerate until cold for slightly thicker cookies, or bake immediately.
Preheat oven to 350F (static) or 325F (convection).
Scoop dough into a piping bag or other freezer weight plastic bag with a small corner cut out. Evenly squeeze buttons of dough out onto a parchment-lined baking sheet, forming rounds approximately ¾-inch wide, spaced 1/2 inch apart.
Bake for approximately 10-14 minutes, or until edges are lightly browned.
Yield: 7 dozen cookies.