Start by making the pastry. Pulse the nuts in a food processor until ground, but don’t overwork them, or they’ll become a paste. Transfer to a bowl.
Place the flour and icing sugar in the food processor. Cube and add 100g of the butter and a pinch of sea salt, then pulse until it’s the texture of breadcrumbs. Add the vanilla extract and a splash of cold water and blitz to form a dough, then pour in the ground nuts and pulse again until it comes together. Take it out, dust with flour, then wrap in clingfilm and chill for half an hour.
Remove the pastry from the fridge and preheat the oven to 180ºC/gas 4. Dust the work surface with flour, and roll out the dough to 5mm thick. Drape it over a deep 25cm fluted tart tin and ease it in. Prick all over with a fork, line with clingfilm and chill for 20 minutes.
Take your tart out of the fridge, and fill with baking beans or rice. Bake in the oven for 12 minutes, then remove the clingfilm and beans and bake for 3 to 4 minutes, or until golden. Remove from the oven and leave to cool.
To make the filling, melt the rest of the butter in a pan over a medium heat until it starts to turn golden. Pour in the rum, grate in some nutmeg, and warm through until light brown in colour. Remove from the heat.
In a bowl, whisk the eggs with the caster sugar until pale, then stir in the buttered rum. Melt and add the white chocolate. Halve the vanilla pod, scrape out the seeds and add to the mixture along with the cream and a good pinch of fine salt, until well combined.
Place the tart case on a baking sheet and pour in the rum filling. Bake in the oven for 30 minutes, checking halfway through that the crust isn’t burning (cover with tin foil if it is).
When the filling is set but is still soft in the middle, it’s done. Take it out and allow it to cool completely.
To make your brittle topping, grease a baking sheet, arrange the nuts on it and set aside. Place the sugar in a pan with 75ml water, and bring it to the boil. Don’t stir the caramel, just swirl it. When it’s a deep golden colour, pour it over the nuts and leave to set.
When the brittle is hard, break it into pieces, sprinkle it over the pie and serve.