Make the pastry by rubbing together the flour, cocoa powder, baking powder, sugar and butter. Add the lemon zest and egg yolk and mix to a soft pastry. Chill for 15 minutes, and then roll out the pastry thinly on a dusted work surface. Use it to line a large 25cm round flan tin. Trim away any excess. Line the tart case with baking parchment, fill with baking beans and bake in the oven for 12-15 minutes at 200`c. Remove the baking beans and paper and cook for a further 10 minutes or until the base of the tart is golden-brown and cooked through. Remove the tart from the oven and set aside to cool. Meanwhile, heat the chocolate and cream in a saucepan set over a medium heat, stirring continuously until the chocolate melts and mixture is smooth and thick. Pour the chocolate mixture into the tart shell and chill in the fridge for 45 minutes, or until set. Remove the tart from the fridge and arrange the fresh raspberries on the top. Dust the raspberries with a little icing sugar and transfer the chocolate and raspberry tart onto a serving plate.