1. Preheat oven to 180C fan forced.
2. In a processor, combine flour and butter until mixture resembles fine breadcrumbs.
3. Add egg and sugar and pulse until mixture is combined.
4. Knead gently into a ball, wrap in clingfilm and refrigerate for 30 minutes.
5. Roll out and line a 23cm fluted tart tin with removable base.
6. Prick base with a fork, line with baking paper and fill with pastry weights.
7. Bake for 10 minutes. Remove pastry weights and bake for a further 10 minutes until the base is cooked.
8. To make the filling, place cream and sugar in a medium saucepan and bring to the boil.
9. Remove from heat and add butter, cocoa and chocolate and stir until combined and smooth. Whisk in milk.
10. Place mixture in a jug and pour through a sieve into the warm tart shell.
11. Leave to set in a cool spot (do not refrigerate).
12. Dust with cocoa and serve.