Preheat the oven to 180°C (160°C fan oven) Gas Mark 4. Roll out the pastry on a lightly floured surface until big enough to line an 18 x 28cm rectangular loose-bottomed tart tin (or use a 22-23cm round one). Line the pastry with foil or greaseproof paper, fill with baking beans and bake for 25 minutes. Remove the paper and beans and cook for 10 minutes more until sandy and golden. Leave to cool.
For the filling, put the cream, 100ml liqueur, dark and milk chocolate chunks in a heatproof bowl over a pan of barely simmering water. Gently melt, stirring very occasionally, until smooth and silky. Pour into the cooled tart case and chill for at least 5 hours, or overnight, until firm.
To serve, remove from the fridge and lift from the tin, melt the white chocolate chunks in the same way as before. Stir in the remaining 2 tbsp of liqueur and drizzle over the tart to decorate. Cut into bars and serve with runny cream or crème fraîche.