Preheat the oven to 200C/gas 6/fan 180C.
Rub the butter into the plain flour, then mix in the golden caster sugar, egg and a pinch of salt. Combine the pastry into a ball and knead it briefly.
Line a 12-hole patty tins, by pressing small walnut-sized balls of pastry into each hole.
Spoon the mincemeat into the pies then make slightly smaller balls of pastry than before and pat them out between your hands to make round lids, big enough to cover the pies
Top the pies with their lids, pressing the edges gently together to seal.
Beat 1 small egg and brush the tops of the pies. Bake for 20 mins until golden. Leave to cool in the tin for 5 mins, then remove to a wire rack. To serve, lightly dust with icing sugar