Gran's mini shortbread tarts with caramel filling

1. beat sugar and butter until light and cream in the bowl of an electric mixer: 
2. Add egg and beat well to combine. 
3. Add flours and mix on a low speed until dough just comes together.
4.  Turn out onto a lightly floured surface and gently form into a ball. Wrap in plastic and refrigerate for 30 mins (my mum ignores this step and rolls it ou immediately, but it's up to you). 
5. Lightly grease four gem scone trays (about 12 "tarts" per tray), or one 23cm loose-bottomed flan tin. On a lightly floured surface, roll the pastry as thinly as possible, and cut out tart shells with a pastry cutter or a glass dusted with plain flour. 
6. Press the pastry rounds into each tart mould, trimming any excess. 
7. Prick the tart shell bases with a fork and return to the fridge for 30 mins.
8. Preheat oven to 180C. Cook tart shells for I0-12 mins or until golden brown. Cool completely in the tins before turning out gently. 
9. Store in an airtight container until required. 

Filling

1. in a mixing bowl, whisk egg yolks, sugar and flour. 
2. Place the milk and vanilla in a saucepan or a high heat. As soon as it comes to the boil, whisk into the egg mixtur 
3. Strain through a fine sieve into a clean saucepan over medium-low heat, stirring continuously until the filling thickens. Remove from heat and whisk in butter. 
4. Pour custard into a dish, cover with plastic wrap and refrigerate until completely