Preheat oven to 180°C (160°C fan) / 350°F. Line the tin with greaseproof paper/baking paper.
Cream together butter and sugar for the shortbread base. Fold in flour and a pinch of salt until dough forms.
Reserve one-third of dough for the crumble topping. Press the remaining dough into the base of the tin and bake for 12-15 minutes until lightly golden.
Spread mincemeat evenly over the warm shortbread base.
Make the crumble by combining reserved dough with extra flour and sugar; fold in optional chopped nuts. Crumble over mincemeat.
Bake for 20–25 minutes until topping is golden.
Cool completely before slicing into 9 generous or 16 snack size squares. Dust with icing sugar if desired.