Cut the roll into eight slices; place on middle rack of a 250F oven and leave until dry, about 20 minutes. In a saucepan, heat 1 1/2 cups of the heavy cream, the milk, and the sugar over low heat, stirring until sugar is dissolved. Add 5 oz. of the white chocolate, stir until melted, and remove from the heat. In a large bowl, whisk the egg and yolks together. Whisk the chocolate mixture into the eggs a little at a time. Tear the bread into 1" pieces, add to the white chocolate custard, and stir to mix. Leave to soak, stirring occasionally, until all the custard has been absorbed by the bread, 1 to 2 hours. Put the mixture into an 8" square, 2" deep baking dish. Put the dish in a slightly large pan and add hot water to come halfway up the sides of the baking dish. Bake the pudding in the water bath at 350F for 45-50 minutes, until the custard is set and the top is golden brown. Serve warm or cold. If you chill it, loosen the sides with a metal spatula and invert the pudding onto a cutting surface. Cut into four squares and cut each square into a triangle. For the sauce, heat the remaining cream in a small pan. Add the remaining 4 oz. white chocolate and melt. If desired, grate some semi-sweet chocolate and sprinkle on top of the pudding.