In a medium bowl, cream together the butter and white sugar until smooth. Beat in the egg yolks one at a time. Split the half of vanilla bean and scrape the seeds; stir the seeds into the butter mixture. Mix in the flour and ground almonds. Divide the dough into two pieces, wrap and refrigerate until firm.
Preheat oven to 180 degrees C. Line baking trays with baking paper. Break off tablespoonful sized pieces of dough then roll them into little ropes about 5cm long. Bend the ropes into a half circle and place them 5cm apart onto the prepared baking trays.
Bake for 10 to 15 minutes in the preheated oven, until lightly browned. Scrape the seeds from the remaining vanilla bean and stir them into the icing sugar. Carefully roll warm biscuits in the vanilla sugar.