Pastry Step 1: In a medium bowl, rub 125g butter into 1 cup flour until mixture resembles coarse crumbs. Using a fork to stir, add the cold water one tablespoon at a time. Stir until the dough begins to form a ball.
Divide dough in half. Roll or press each half into a 30cm x 8cm rectangle. Place them 8cm apart on an unprepared baking tray.
Pastry Step 2: In a saucepan, bring 125g butter and 1 cup water to a rolling boil. Lower heat and stir in the flour immediately with a wooden spoon. Continue to stir vigorously until the dough forms a tight ball. Remove from heat. Stir in eggs one at a time then stir in the almond essence.
Spread dough equally over the two rectangles already on the baking tray. Bake for 10 minutes before lowering the oven temperature to 165 degrees C. Cook for 15 to 20 minutes more. The top should be crispy and brown.
Icing: Beat icing sugar, melted butter and vanilla until smooth. Spread over the almond puff when warm, not hot, and sprinkle with sliced almonds. Cut into strips and serve.