With a mesh strainer, sift the flour into a large bowl and add the powdered sugar and salt. Mix everything well together.
Add the ice cold or frozen cubes of butter and with your hands (or using a food processor), massage the butter into the flour mix until you get a crumbly shortcrust pastry.
whisk the egg in a cup and add to the bowl. Add the orange zest and with a fork, mix the egg and zest into the pastry dough.
put the crumbs onto the kitchen countertop and knead into an uniform dough. Form the dough to a ball.
Put the dough ball between two large sheets of parchment paper. With a rolling pin, roll out the dough into a disk about 3-4mm thick. Put the disk into the fridge (laying flat) for 1 hour or longer.
grease the muffin pan with some melted butter. With a cookie cutter, cut out round disks (a little larger than the opening of the tartlet pan) from the cold and hard disk of dough.Gently press the disks into the muffin pan, removing any air bubbles. Try to work safely but quick at this step, as the dough quickly becomes soft and brittle again. With a fork, prick the bottom of the pastries. Put the muffin form into the fridge for 30-60 minutes. Cover each tartlet shell with a piece of parchment paper and add some baking weights (or dry rice, beans etc.). Blind bake the shells in the oven at around 170°C / 340°F for 15minutes.
Take the Muffin pan out of the oven again, remove the baking weights and bake again for another 15 minutes until the shells turn golden. Take the shells out of the muffin pan and let them completely cool down on a cooling rack