Preheat oven to 350°F. Lightly grease two baking sheets with butter. Place the almonds in a food processor bowl and process for about 15 seconds until finely ground. Transfer the ground almonds to a shallow bowl and set aside.
In a large bowl, cream the butter and brown sugar together until light and creamy. With an electric mixer, beat in the egg yolk until well blended. Beat in the almond and vanilla extracts. At low speed, add the oat mixture, mixing just until blended.