Melt the chocolate in a bowl over a pan of simmering water (see
Beat the egg whites in a separate bowl until they are stiff. Use a large metal spoon to carefully fold one-quarter of the whites into the chocolate mixture to lighten it, then fold in the remainder.
Grease and line a 33 x 23 cm (13 x 9 inch) Swiss roll tin. Turn the mixture into the tin, spreading right into the corners. Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 15 minutes until just firm to the touch. Remove from the oven and cover with a damp clean tea towel. Leave to cool for 1–2 hours.
Whip the cream until softly peaking, then stir in the chocolate sauce, reserving 2 tablespoons. Stir in the icing sugar.
Sprinkle a large piece of baking paper generously with caster sugar. Turn out the roulade on to this. Spread the banoffee toffee over the roulade to within 1 cm (½ inch) of the edges, then spread over the chocolate cream. Roll up, starting at a short end. Lift on to the serving plate. Arrange the chocolate leaves on the top, securing with the reserved chocolate cream.