Preheat the oven to 180ºC/350ºF/gas 4. Line a 12-hole muffin tray with paper cases.
Melt the butter in a small pan over a medium heat, then leave to cool slightly. Meanwhile, sieve the flour and xanthan gum into a bowl, then stir in the ground almonds and sugar. Add the melted butter and vanilla extract, then fold through.
In another bowl, separate the egg whites (keep the yolks for another day). Whisk the whites well for 1 to 2 minutes, or until they start to froth. Then, using a metal spoon, stir the egg whites into the flour mixture a little at a time – you want to keep it as light and airy as possible so make sure you don’t stir too much. Roughly chop and add the white chocolate, along with half the blueberries, then stir briefly to combine.
Divide the mixture between the paper cases, then sprinkle over the remaining blueberries. Place on the middle shelf of the hot oven for 15 to 20 minutes, or until golden and an inserted skewer comes out clean. Leave to cool on a wire cooling rack, then serve.