First make the pistachio butter: combine all the ingredients in a food processor and blitz until smooth and creamy. This will take 10 minutes or thereabouts, so be patient and bear with it.
In a mixing bowl, whip the cream until it begins to stiffen, sift in the icing sugar and whip again until stiff peaks form, taking care not to over-whip. You want the cream soft and pillowy, rather than clumpy.
Discard the paper wrapping around the panettone, then cut it horizontally into 3 layers. Arrange the bottom tier on a serving dish or cake stand, spread with half the pistachio butter, spoon over half the whipped cream and top with the second tier of panettone. Repeat this process a second time with the last of the pistachio butter and cream, then crown it with the top of the panettone.
In a large, spotlessly clean mixing bowl, whisk the egg whites with an electric mixer until they begin to froth. Add the caster sugar 1 tbsp at a time, still whisking, until you have a white, glossy meringue with stiff peaks. Whisk in the vanilla extract.
Spoon the meringue over the tower of pistachio-and-cream-laden panettone, using the back of a spoon or a butter knife to spread it out evenly, covering the entire cake. Create swirls and peaks with the spoon, then use a blowtorch to gently brown the surface, so it caramelises.
Store in the fridge: the cake can be made up to 1 day in advance.