Place the sugar, golden syrup and butter in a saucepan over a medium heat and bring to the boil, stirring occasionally. Pour in 200 ml (7 fl oz) of the evaporated milk and cook for 3 minutes.
Place the custard powder in a medium bowl and stir in the measurement water to form a paste, then gradually stir in the remaining evaporated milk until smooth. Pour into the hot sauce and cook, stirring continuously, for 2–3 minutes until the custard is thick and smooth. Stir in the rum, if using.
Arrange the bananas in a dish and sprinkle over the pecans. Pour over the hot custard and serve immediately.