TO make pastry, sift flour and salt. Place in a food processor with the cubed butter. Pulse until mixture forms fine crumbs. (If you don't have a food processor, keep the butter cold and in one piece and grate on the coarse side of a grater, directly into the flour. Mix with the fingertips until mixture forms fine crumbs. Avoid overworking.)
Add beaten egg, a little at a time, until pastry starts to bind together. (Depending on the size of the egg and dryness of the flour, you may or may not need all the egg. However, if mixture looks a little dry, add a few drops of cold water.) Knead pastry very quickly and lightly until smooth. Wrap with greaseproof paper or cling film and refrigerate for 15 to 20 minutes before rolling out.
Roll pastry out thinly and evenly on a lightly floured working surface and line a 20cm shallow tart pan or 15 to 18 tartlet tins. Trim and fluke edges and prick the bases with a fork. Chill pastry in the fridge for 15 minutes.
Preheat oven to 190°C. Line the pastry with greaseproof paper and fill with dried beans or (uncooked) rice. (This prevents the pastry from rising and bubbling.) Bake in the preheated oven (15 minutes for the large tart, 8 to 10 minutes for the tartlets) before removing the greaseproof paper and beans. Return to the oven for another 5 to 7 minutes to dry out the bases, before spooning in the filling.
While the pastry is in the oven, prepare filling. Cream butter and sugar together, stir in the eggs, one at a time (if it curdles, it isn't a tragedy!).
Add the durian flesh, grated coconut, salt and coconut milk, stirring well. Fill the partially-cooked pastry case (or tartlets) with the filling. Turn the oven temperature down to 160°C and bake until the filling of the tart or tartlets is golden brown (about 15 minutes for the tartlets and 25 to 30 minutes for the large tart).