Start your biscuits the night before. Preheat the oven to 100ºC/gas ¼.
Arrange the almonds on a baking tray and toast in the oven for 5 minutes. Take them out (turn off the heat) and leave them to cool.
Put the nuts into a food processor with 2 tablespoons of the icing sugar and whiz until the almonds are ground to the texture of semolina. Don’t over-blitz, or the oils in the nuts will make it sticky.
Sieve another 180g of icing sugar and the cocoa into a bowl, then stir in the ground almonds. Set aside.
Melt the chocolate in a small pan over a low heat, then leave to cool slightly.
In a separate bowl, whisk the egg whites with an electric whisk until they form stiff peaks. Halve the vanilla pod lengthways, scrape out and add the seeds to the egg whites. Beat in the almond extract.
Gently fold the egg whites into the almond mix, then stir in the melted chocolate. Leave in the bowl, cover with clingfilm and chill overnight.
The next day, preheat the oven to 160ºC/gas 2½, and line 2 baking trays with greaseproof paper. Take the dough out of the fridge and leave it at room temperature for 10 minutes to soften up.
Dust the work surface with a little icing sugar and, using your hands, roll the dough into a sausage. Cut it into about 25 pieces and shape them into balls, each about 4cm in diameter.
Scatter the remaining 250g of icing sugar onto a plate. Take each ball and roll it in the sugar, making sure they’re all coated in a thick layer.
Put them on the baking trays, squashing each one very lightly, so it’s 1cm thick, and leaving some room between each one.
Bake the biscuits in the oven for 18 to 20 minutes, or until crinkled on the surface. They’ll still be soft, but don’t worry, they will set.
Leave to cool on the trays for 5 minutes then transfer to a wire rack to cool completely. Store in a tin to keep moist.