Important: Have all ingredients as cool as possible.
Sift the flour and cream of tartar together 3 times. Divide the butter into 5 equal pieces.Cut one piece of the butter into the flour and rub in till it resembles fine bread crumbs.Alternatively, put the flour and the butter into your food processor and process till fine. Beat the egg yolk and lemon juice together and add ice water to make 1 cup. Mix this into the flour with the blade of a knife till a dough is formed. Knead lightly and refrigerate (covered in plastic wrap) for 10 minutes.
Meanwhile, grate each remaining piece of butter and put in the freezer. It is best to freeze each piece of butter beforehand and grate using a food processor.
Roll out the dough into a rectangle.Fold the 2 long sides in thirds so that they overlap. Now do the same with the short sides.(if you're unsure, check a pastry book for the folding).Roll out to a rectangle again. Sprinkle with 1 portion of the grated butter - if it sticks together in lumps just regrate over the pastry.leave a 2cm margin around the edges. Sift the cornflour/cream of tartar liberally over the mixture. Fold as before and roll. Repeat till all the butter and cornflour is used up. Then roll and fold again twice (without any butter or cornflour).
To prevent the dough sticking while rolling lightly flour the rolling pin and board with cornflour. The best surface to use is a marble pastry board if you have one otherwise any large flat surface will do. Roll very lightly so the butter blends into the pastry - no lumps of butter should break through(this is why we grate the butter). If at any time the pastry becomes too sticky, refrigerate for a few minutes and continue.
Once finished, refrigerate for 30 minutes, then roll out again and use as required. Glaze with a little milk before baking.
To bake preheat the oven to 250° Celsius.Put the pastry in for 5 minutes, then turn down the heat to 220° Celsius. Bake for 10 minutes then turn down to 180 degrees Celsius and bake till done.