1. Fit your food processor with the dough blade, add all the 'puff pastry' ingredients and process until the flour clumps together in large pieces and comes together.
2. Roll out the dough on a floured surface with a floured rolling pin into rectangle shape and sprinkle cold unsalted butter (150g) onto the dough (see photo).
3. With the short border of the rectangle closest you, fold the dough into thirds: right and left edges to be folded in (see photo). The butter should be enclosed in the dough. Now roll it out again into a big rectangle with the rolling pin and fold it again.
4. Wrap the dough in plastic and chill for about 30 minutes or until firm.
5. Repeat steps #3 and #4 twice more. The dough should be smooth by now. Chill it in plastic again for 30 min.
6. Now make the filling : Beat the softened butter and sugar together until light and fluffy, add the beaten egg. Mix well. Stir in the almond powder and chestnut cream. Mix well to combine evenly. Set aside.
7. In a medium bowl, add egg and sugar. Mix well. Sift the cornstarch in the bowl to combine them. Mix well. Pour the milk gradually onto the egg mixture. Mix well until combined evenly. Strain the mixture to saucepan using a strainer and cook over moderate heat, stirring constantly, until sauce thickens, using a wooden spoon or a spatula. When the sauce thickens enough, then remove from the heat. Let it cool to room temperature.
8. When the cream has reached room temperature, then mix with almond cream which you just made (step 6). Keep it in the fridge until the pastry is ready.
9. Preheat your oven to 180C degree. Divide the puff pastry in half, roll out each piece and cut into about 25cm circle. Place a circle dough on a baking sheet and prick bottom of pastry thoroughly with a fork. Then spread the filling which you made at step #6 evenly within about 2cm of the edges (Don't forget to bury your fève in the almond filling). Brush the edges of the pastry with water. Cover with the other circle of the pastry and press it down to seal the edges very well. Draw any design you like on the top of the pie using a knife, then brush the pie with beaten egg.
10. Bake for 50 min. Then heat up to 200C degree and bake for another 10 mins or until nicely browned on top.
NOTE : If you just want "almond filling", then make double quantities of almond filling:)