Preheat oven to 200C. Toss rhubarb, 150gm caster sugar, juices, vanilla seeds and cinnamon in a roasting pan and roast until tender (25-30 minutes), discard cinnamon and set aside.
Meanwhile, for custard, whisk yolks and sugar in a bowl to combine. Bring cream, milk, brandy and vanilla bean to the simmer in a saucepan over medium-high heat, then pour into yolk mixture while whisking continuously. Return mixture to pan and stir continuously until it thickly coats the back of a spoon (4-5 minutes). Strain into a bowl, cover and refrigerate to chill.
Reduce oven to 180C. Butter six 350ml shallow ovenproof bowls or ramekins, spoon a little rhubarb and syrup into each, then pour 60ml custard into each. Beat butter, brown sugar, rinds and remaining caster sugar in a bowl until pale (1-2 minutes), beat in yolks and buttermilk, then fold in flour, hazelnut meal and baking powder. Whisk eggwhites with a pinch of salt in a bowl to soft peaks, fold into batter, then spoon into ramekins. Bake until golden and centre springs back when gently pressed (25-30 minutes). Scatter with hazelnuts and serve hot with chilled brown sugar-brandy custard and extra rhubarb.