Roll the pastry out thinly on a lightly floured surface until a little larger than a buttered 22 cm (8½ inch) loose-bottomed fluted tart tin. Line the tin (see
Bake the tart blind (see
Heat the butter and sugar in a saucepan until the butter has melted and the sugar dissolved. Add the condensed milk and cook over a low heat, stirring for 4–5 minutes until just beginning to darken and smell of caramel. Pour into the tart case, spread evenly and leave to cool.
Melt the dark and white chocolate in separate bowls set over two small saucepans of just simmering water, making sure that the water does not touch the base of the bowls.
Remove the tart from the tin and transfer to a serving plate. Drizzle random spoonfuls of plain dark chocolate over the pie. Cool for 10 minutes, then drizzle random spoonfuls of melted white chocolate over the dark layer. Leave to set. Cut into thin wedges to serve.