Preheat the oven to fan 180C/conventional 200C/gas 6. Generously butter six non-stick friand or muffin tins. Melt the butter and set aside to cool.
Sift the icing sugar and flour into a bowl. Add the almonds and mix everything between your fingers.
Whisk the egg whites in another bowl until they form a light, floppy foam. Make a well in the centre of the dry ingredients, tip in the egg whites and lemon rind, then lightly stir in the butter to form a soft batter.
Divide the batter among the tins, a large serving spoon is perfect for this job. Sprinkle a handful of blueberries over each cake and bake for 15-20 minutes until just firm to the touch and golden brown.
Cool in the tins for 5 minutes, then turn out and cool on a wire rack. To serve, dust lightly with icing sugar.