In a bowl, mix the softened butter and light brown sugar until creamy. Add the eggs, one at a time, mixing well after each addition. Stir in the vanilla extract and orange zest. Fold in the sifted plain flour, baking powder, ground almonds, and mincemeat, mixing until just combined. Finally, add the broken shortbread biscuits and chopped white chocolate, stirring gently. Pour the mixture into the prepared baking tray and smooth the top. Bake for 25-30 minutes or until a skewer comes out with a few moist crumbs. Let cool in the tray before slicing into blondies.