Preheat oven to 190°C (375°F/Gas 5).
In a Robot Coupe or food processor blitz the almonds, marzipan, vanilla seeds and flour to a fine even consistency and set aside.
Put the eggs, extra eggwhites and sugar in an electric mixer and whisk on high speed until thick and pale. Fold the dry mixture through the egg mixture.
Line two 25 × 35 cm (10 × 14 inch) flat trays with silicone paper or baking paper, divide the combined mixture between them and spread it out to an even 1 cm (⅜ inch) thickness. Bake in the oven approximately 6–8 minutes until golden brown. Sprinkle with fine caster sugar. Transfer the cake, still on the paper, to a cooling rack and allow to cool completely. Put the cake in an airtight container until required.