Heat oven to 180C/fan 160C/gas 4. Grease then line the base and sides of 2 x 20cm sandwich tins with greaseproof paper. Sieve the flours and baking powder together.
Use electric hand beaters to whisk the egg whites until stiff, then briefly whisk in the sugar. Beat the egg yolks quickly, then whisk into the whites. Fold in the dry ingredients using a large metal spoon, then spoon the mixture into the tins and level the tops. Bake for 18-20 mins until risen, light golden and a skewer inserted into the middle comes out clean. Cool in the tins for 10 mins, then gently remove and leave to cool completely.
Mix the mandarins and fromage frais together. Peel away the greaseproof paper, sandwich the cakes with the mandarin mix, then dust with the icing sugar to serve. Best eaten on the day it’s made.