1/4 cup butter
1 cup brown sugar
1 teaspoon baking soda
1/2 teaspoon salt
1 egg yolk
1 teaspoon vanilla extract
Preheat oven to 350 degrees F (175 degrees C). Grease cookie sheets.
In a high-speed blender, cream together butter, brown sugar, baking soda and salt until smooth. Beat in egg yolk and vanilla. When mixture is frothy, press 1/2 cup of mixture onto the bottom and 1/2 cup on the top of a greased cookie sheet. (Note: Cookies are best stored in a cool dry place to prevent cracking.)
Bake for 10 to 12 minutes in the preheated oven, until edges are golden brown. Remove from cookie sheet to wire rack to cool completely.