- In a large bowl, with an electric mixer, cream the butter and sugar together on low speed.
- Beat in the egg yolk.
- Place a spoonful of the butter mixture in a cup.
- Split the vanilla bean length-wise, open it to expose the tiny seeds and, with a small spoon or blunt knife, scoop them out and transfer to the spoonful of batter mixture, mix well.
- Add the vanilla bean mixture to the butter mixture and blend well.
- In a small bowl, whisk together the flour and salt, and add to the butter mixture on low speed; combine well.
- Shape the dough into a log 9 inches long.
- Wrap in wax paper or plastic wrap and twist the ends closed.
- Chill until firm, about 2 hours or overnight.
- Preheat the oven to 350° F.
- Cut the chilled dough into ¼ inch thick slices and place 1 inch apart on ungreased baking sheets.
- Bake in the center of the oven for about 12 to 15 minutes, or until the cookies are very light golden brown at the edges.
- Let the cookies cool on the baking sheet for a few minutes, then transfer them to a wire rack to cool completely.
- Store in an airtight container.