Place the sifted flour, salt and butter in a bowl and use your fingertips to rub the butter into the flour until the mixture has an even, granular texture. Add the powdered sugar and blend. Sprinkle with the water, mix gently and gather together to form a ball. Cover in plastic wrap and refrigerate for 15 minutes.
Butter a 20.5 cm (8 in.) tart pan with a removable bottom.
Roll out the dough to a thickness of .5cm and place it over the pan without stretching. Press well into the bottom and up the sides, then roll the rolling pin firmly over the top to trim the excess. Prick with a fork and bake for 10 minutes.
Prepare the filling:.
In a saucepan over medium- low heat, make a roux by melting the butter and stirring in the flour with a wooden spoon. In a separate saucepan, bring the milk, salt and sugar to a boil.
Add the hot milk to the roux a little at a time, whisking constantly until the mixture is smooth. Continue whisking and cooking over low heat for 5 minutes, until thick.
Remove from heat and add the egg yolks and powdered almonds, whisking briskly. Do not allow the filling to boil after the egg yolks are added, or it will separate.
Pour the filling into the pre-cooked pie shell. Sprinkle the sliced almonds over the top and bake for 20 minutes, or until the top is nicely browned. Cool completely and sprinkle with powdered sugar, if desired.