For the candied hazelnuts, line a baking tray with baking paper. Put the hazelnuts, sugar and butter into a small frying pan. Cook over a low heat until the sugar has melted, swirling occasionally but not stirring.
Turn up the heat slightly and continue to cook and swirl until the caramel has turned a deep golden-brown. Immediately tip the hazelnuts on to the baking tray and separate them out. Leave to harden and break up a little if you want them in smaller pieces.
For the chocolate sauce. Put the chocolate, cream and golden syrup into a saucepan and melt on a very low heat, whisking to combine. Whisk in the butter and rum, and keep warm while you prepare the bananas.
For the banana splits, gently melt the butter in a large frying pan. When it starts foaming, add the bananas, then sprinkle over the sugar and a pinch of salt. Turn up the heat to medium-high and cook the bananas, turning every so often, until they are well coated in caramelised, buttery sugar.
Divide the bananas between four bowls and top with ice cream, whipped cream, the candied hazelnuts and the chocolate rum sauce.