Beat the egg whites with the whisks of the hand mixer until very stiff. Stir in the almond flavour and gradually add the finely sieved icing sugar. Fold the ground almonds into the egg whites.
Pour into a piping bag with a medium-sized punched spout and squirt small heaps onto the wafers. Cut the pistachios in half and place 3 halves of each wafer on the meringue mixture while it is still moist. Place the amaretti on a baking tray and leave to dry overnight in a cool room.
Bake in the preheated oven (electric cooker: 150 °C/ gas: level 1) for approx. 15 minutes. Allow to cool and layer in a pastry tin. This way they stay juicy longer. Results in approx. 90 pieces.