1. Sieve the flour, icing sugar and cocoa powder not a large mixing bowl. Add the butter and using your fingers rub the mixture until it resembles fine breadcrumbs. Add the egg yolk, lemon juice and water and bring the mixture together to form a dough. Wrap in cling film and leave to rest in the fridge for 30 minutes.
2. Heat your oven to 200°C/gas 6.
3. Once the the pastry has rested, dust your work surface with flour. Roll out the pastry to approximately 3mm thick and use it to line a 25cm tart tin. Leave any excess pastry overhanging the edge of the tin. Prick the base with a fork. Line the pastry case with baking parchment then fill with baking beans or uncooked rice. Bake blind for 15 minutes, then remove the parchment and baking beans and return to the oven for a further 8-10 minutes until the pastry is dry and cooked through. Using a small knife trim away any excess pastry from the edge. Leave the base to cool completely.
4. To make the filling place the milk, cream, sugar, cocoa powder, chocolate, cornflour and butter into a saucepan. Set the pan on a medium heat and using a whisk stir until the chocolate and butter have melted and you have a smooth, glossy and thick mixture. This will take around 10 minutes. Don’t rush this or have the heat too hot as it can burn the chocolate.
5. Pour the mixture into the cooled tart case. Cover with cling film to prevent a skin forming. Leaving to cool then place in the fridge to set.
6. Remove from the fridge for 2 hours before serving. This allows the filling to soften and come to room temperature. Decorate with mini chocolate eggs and a dusting of icing sugar. Serve with cream.