Preheat the oven to 190°C/fan 170°C/gas 5, and line a 12-hole muffin tray with paper cases. Whisk the egg whites in a bowl using a hand-held electric whisk, or place in an electric food mixer and whizz, until the egg whites are foamy but not holding stiff peaks.
Sift in the icing sugar and flour, then add the ground almonds and melted butter and stir just to combine. Divide the mixture between the muffin cases, filling each three-quarters full, then add two raspberries and two blueberries to the centre of each cake, pressing the fruit lightly into the batter. Bake for 20-25 minutes or until a light golden colour and slightly springy to the touch.
Remove the friands from the oven and leave in the tin for 5 minutes, then remove from the tray, place on a wire rack and allow to cool completely. Arrange the cakes on a plate and dust with icing sugar to serve. (If not eating immediately, store them in an airtight box, where they will stay wonderfully moist for up to three days.)