Roll out each piece on a lightly floured surface to a square, a little larger than 25 cm (10 inches).
Cut a 25 cm (10 inch) round from each piece of pastry, using a large plate r a saucepan lid as a guide.
Place one of the rounds on a baking sheet.
Beat the butter until it’s very soft, then beat in the icing, sugar, egg yolks and rum.
Mix in the ground almonds until well combined.
Spread the almond filling in the center of the pastry round on the baking sheet, leave a border of about 2.5 cm (1 inch) all around.
Brush the pastry with a little cold water, then place the second pastry round on top to enclose the filling.
Press the edges firmly together to seal.
Chill the pastry for at least 30 minutes.
Make the glaze by lightly beating the egg and the icing sugar together.
Lightly flake the edge of the pastry by tapping it gently with a small knife.
Decorate the edge by fluting it all the way around with the back of a knife.
Brush the top of the pastry with the beaten egg glaze. (do not allow the glaze to run down the sides of the pastry as this will prevent it from rising evenly.)
Mark the pastry with a small knife by making long curved lines, scoring about halfway through the pastry. You should start at the center and end at the edge.
The lines should be about 0.5 cm (1/4 inch) apart and it should look like a spiral pattern.
Make a small hole in the center of the pastry to allow the steam to escape.
Bake at 230ºC (450ºF) mark 8 for about 20-25 minutes until it’s well risen and golden brown.
Remove from the oven and cool on the baking sheet.
Serve the pastry when it has cooled down to room temperature.
