Heat the oven to 200°C/Gas 6. Melt the butter and allow to cool, then use 1 tbsp to coat 10 muffin tin moulds or individual oval baking moulds measuring 5x 10cm.
Sift the icing sugar and flour into a bowl, and mix in the ground almonds. Lightly beat the egg whites with a fork, then fold them into the dry ingredients. Add the cooled, melted butter and lemon rind, and mix well.