First make the ice cream. Heat the sugar in a large, heavy-based frying pan until completely melted.
When the sugar starts to turn a light caramel colour, add the bananas and shake the pan gently to coat. Cook for two minutes until caramelised then remove from the heat and pour in the rum.
Leave to bubble for a minute then pour in the cream and vanilla. Stir to combine and scrape up any bits of caramel from the bottom of the pan then pour into a liquidiser and blend until smooth. Churn in an ice cream machine until set.
When set, spoon the ice cream into a 500ml bowl lined with cling film and return to the freezer. Preheat the oven to 180 degrees Celsius or 350 degrees Fahrenheit and grease and line a 20cm loose-bottomed cake tin.
Cream the butter and sugar with electric beaters until pale and fluffy. Beat in the eggs, one at a time, then add the ground almonds and beat until smooth. Sift in the flour and baking powder and stir in gently, keeping as much air in the mixture as possible.
Fold in the sour cream and vanilla extract then spoon into the lined tin. Bake for 20 to 25 minutes until a skewer inserted into the middle of the cake comes out clean.
Transfer to a wire rack to cool. When the cake is cool, use a 500ml bowl (the same as used for moulding the ice cream) as a template to cut a disc from the centre of the cake.
Increase the oven temperature to 200 degrees Celsius or 400 degrees Fahrenheit. Line a baking tray with greaseproof paper and lay the sponge disc on top. Turn the ice cream out, lay on top of the sponge and return to the freezer whilst you make the meringue.
Put the hazelnuts and 2 tablespoons of the sugar into a food processor and blitz until fine. Whisk the eggs whites with a pinch of salt to stiff peaks then gradually whisk in the sugar until stiff and glossy.
Fold in the ground hazelnuts, keeping as much air in the mixture as possible. Spread the meringue over the ice cream and sponge in an even layer, making sure there are no gaps or holes.
Bake for 10 minutes until crisp. Drizzle the baked Alaska with the melted chocolate, sprinkle with chopped hazelnuts and serve immediately.