Pre-heat the oven to 200°C (392°F)
With a whisk, cream the butter and mix in the ground almonds and flour.
Beat the three egg yolks and add to the almond mixture, a little at a time.
Incorporate rum and vanilla (if using).
Place one pastry round on a baking sheet. Spoon the almond and egg mixture into the centre. Insert the fève into the mixture about two inches from the edge.
Mix the egg whites with two teaspoons of water. Use a pastry brush to wet 1/2″ edge around pastry round with egg mixture to form a seal. Place the other pastry round on top and gently pinch edges. Then with the pastry brush, wet the top of pastry with egg wash mixture to give it a shiny finish.
Score an inner circle about half an inch from the edge, ensuring that the cut does not go all the way through the pastry. In the same way, very lightly score a pattern or a few lines on the top to let the steam out while baking.
Bake about 25-30 minutes or until the top turns golden and the edges puff up.
Cool on a wire rack.
Sprinkle with icing sugar to serve.