In a small saucepan, heat water with salt, and stir until salt is dissolved. Remove from heat and let cool.
Meanwhile, combine sifted flour and butter in a food processor bowl until mixture resembles breadcrumbs.
Add salted water slowly to the mixture and pulse until a soft ball forms and the dough separates from the bowl. Wrap tightly with cling film and chill in the fridge for 30 minutes.
Preheat the oven to 200 C / Gas 6. Cover a baking tray with aluminium foil and grease. Set aside.
Roll out pastry on a floured surface to a 2mm thickness. Cut into 10cm circles with a pastry cutter or sharp knife. Put one big dollop of dulce de leche in the centre of each circle. Fold the pastry over in half to enclose the filling. Use a fork to press and seal the edges.
Place empanadas on the prepared baking tray, then brush with beaten egg and sprinkle with caster sugar.
Bake in preheated oven for 10 minutes, or until pastry is golden.
Dulce de leche
Dulce de leche is essentially condensed sweetened milk reduced to a caramel, much like what is used in a banoffee pie. You can make your own or simply buy tinned caramel, such as that available from Carnation®.