Extract the vanilla seeds by making a slit down the side of the pod, spreading both sides out and scraping the inside with the back of your knife.
Preheat the oven to 180 °C.
Mix the ground almonds and sugar together in a bowl.
Lightly beat the egg whites and mix in the vanilla seeds.
Stirring constantly, add the egg whites very gradually to the almond mixture a little at a time. The resulting mixture should be fairly firm and stiff.
At this point you can add the optional few drops of rose water.
Spoon the mixture into a pastry bag with a star-shaped tip and pipe about 20 small mounds of mixture onto a greased baking sheet.
Decorate each one with half an almond and bake in the oven until the peaks are golden brown.
The cookies will keep for quite a long time in an airtight container.