For the pastry, sift the flour, cocoa powder, icing sugar and a pinch of salt into a food processor. Add the butter and whizz until the mixture looks like fine breadcrumbs. It works just as well if you do it by hand.
Beat the egg yolk briefly with the water. Add to the food processor and whizz until the mixture starts to stick together. Turn out onto a lightly floured surface and knead briefly until smooth. Shape into a flat disc, wrap in cling film, then chill in the fridge for 15 minutes.
Remove the pastry from the fridge and roll it out thinly on a flour-dusted surface. Use it to line a 25cm (across the top), 4cm deep, loose-bottomed tart tin. Trim the pastry edges, then chill in the fridge for 20 minutes.
Put a baking sheet onto the middle shelf of the oven and preheat to 200°C/fan180°C. Line the pastry case with a sheet of crumpled baking paper and a thin layer of baking beans or rice, then blind bake on the preheated baking tray for 15 minutes. Remove the paper and beans/rice and blind bake for 5 more minutes. Remove from the oven and leave to cool and crisp. Turn the oven down to 170°C/fan150°C.
For the filling, put the milk, cream and sugar into a pan and slowly bring to the boil, stirring gently. Take off the heat, add the chocolate squares and stir until smooth. Cool slightly, add the eggs and vanilla extract, then mix together well.
Scatter the raspberries in the tart case. Pour over the filling, carefully slide the tart back onto the baking sheet in the oven, then bake for 20-30 minutes or until the filling is cooked but still slightly wobbly. It will continue firming up as it cools. Remove and leave to cool somewhere cold (but not in the fridge!) until set. Serve cut into thin slices with more fresh berries and cream.
Recipe from “The Hill Street Grocer”