Preheat the oven to 200°C, fan 180°C, gas 6. Blind bake the tart case: unroll the pastry and use to line the base and sides of an 18cm round fluted tart tin. Lay a sheet of baking paper over the pastry and fill with baking beans or scrunched-up kitchen foil. Bake for 10 minutes, then remove the beans or foil and paper, and bake for 7-10 minutes until the base is a pale golden colour.
While the pastry case is baking, prepare the brownie mixture. Put the chocolate, butter and caster sugar in a large pan and heat gently, stirring every so often, until melted. Transfer to a large bowl and leave to cool for 10 minutes.
Remove the pastry case from the oven and turn the temperature down to 180°C, fan 160°C, gas 4.
Whisk the egg yolk and flour into the brownie mixture. In a separate clean bowl, whisk the egg white to soft peaks, then gently fold into the chocolate mixture. Fold in the mincemeat. Spoon into the pastry case and bake for 45 minutes.
Cut out a 6cm star from greaseproof paper. Just before serving, place the star in the centre of the tart, dust over the icing sugar, then remove to reveal a star.