This is from Green and Blacks Chocolate Book.

Chocolate-Crusted Lemon Tart

Ingredient:
PASTRY
· 175g (6oz) plain flour
· 25g (1oz) cocoa
· pinch of salt
· 25g (1oz) icing sugar
· 125g (41/2 oz) unsalted butter, chilled and diced
· 1 large egg yolk
· 2 tablespoons cold water
FILLING
· 75g (3oz) dark chocolate,
· minimum 60% cocoa solids, grated
· 3 juicy unwaxed lemons
· 150g (5oz) caster sugar
· 4 large eggs
· 150ml (5oz) double cream
· icing sugar to sprinkle
Instruction:
PASTRY
1. To make the pastry, sift together the flour, cocoa, salt and icing sugar. Rub in the cold butter using a food-processor or your fingertips until the mixture resembles fine breadcrumbs.
2. Mix the egg yolk with the water, and add to the mixture to make a dough. You may need a little more water. Gather the pastry into a ball, wrap it in grease-proof paper and chill in the fridge for about 1 hour.
3. Roll out the pastry from the centre and away from you, then back to the centre and down towards you, using your weight to push down on it to avoid stretching it. Line the tart tin.
4. Prick the pastry base with a fork in several places and chill for at least 2 hours or overnight.
5. Preheat the oven to 200°C/400°F/gas mark 6.
6. Line the pastry case with foil and baking beans and bake blind for 15 minutes, then remove the foil and beans and bake for a further 5 minutes. (Be careful not to overbake as the chocolate pastry can quickly develop a bitter taste.) Remove the tin from the oven and reduce the temperature to 170°C/350°F/gas mark 3.
7. While the pastry is still hot, scatter the grated chocolate evenly over the base and then leave to cool.
FILLING
1. To make the filling, finely grate the zest from the lemons into a mixing bowl. Squeeze and strain the lemon juice and add it to the zest with the sugar. Whisk until the sugar has dissolved, then whisk in the eggs and the cream until the mixture is smooth.
2. Pour the filling into the cooled pastry case and carefully return it to the oven. Bake for 30–35 minutes until just set. Remove from the oven and leave on a wire rack to cool completely before removing from the tin.
3. Dust with icing sugar before serving.