Melt the butter and leave to cool (see
Whisk the egg whites in a large clean bowl with a hand-held electric whisk until frothy and increased in volume but not peaking. Add the sugar, flour and ground almonds and stir in until almost combined. Drizzle the melted butter over the mixture, then stir together gently until just combined.
Divide the mixture evenly between the holes of a greased 12-hole muffin tray, then scatter the strawberries, cherries or cranberries on top. Bake in a preheated oven, 200°C (400°F), Gas Mark 6, for about 20 minutes until pale golden and just firm to the touch. Leave in the tin for 5 minutes, then transfer to a wire rack to cool. Serve sprinkled with caster sugar.