Beat butter until soft. Gradually add sugar, blending until light and creamy.
Mix in beaten egg, then add vanilla.
Soft together flour, salt, and baking powder, then stir into the batter mixture.
When smooth, form into a 2" diameter roll on wax or parchment paper, then wrap securely.
Chill the roll for 12 to 24 hours in refrigerator, or keep in freezer.
To bake, slice ¼" disks off the end and place on a greased cookie sheet. Bake ar 400° for 8-10 minutes.