Mix the dry yeast with some of the luke warm milk en stir until dissolved. Place buckwheat and the flour together in a bowl and make a small circle in the middle. Add the yeast mixture into it. Add the milk and stir until you have a smooth batter. Add the eggs, salt and vanille sugar and stir through. Leave to stand and rise for about 45 minutes. Heat the poffertjespan and add a bit of butter into each hole. Fill halfway with batter and first bake one side until you can see the top dry out a little. Turn the poffertjes around with a small fork and bake the other side until cooked and golden brown. Serve the poffertjes with butter and icing sugar