Separate the egg. Mix butter in small pieces, sugar, egg yolk and flour to a smooth dough. Wrap in foil and chill for about 30 minutes. Then roll out on a lightly floured work surface approx. 5 mm thick. Cut out toadstools (5 x 4.5 cm) and place them on 2-3 baking trays lined with baking paper. Knead the dough again, roll out and cut out. Bake sheet by sheet in a preheated oven (electric range: 200 °C/ convection oven: 175 °C/ gas: level 3) for 8-10 minutes, allow to cool.
To decorate, lightly beat the egg whites, add icing sugar and stir in lemon juice. Set half the icing aside. Divide the rest of the icing in thirds. Colour orange, pink and red. Fill all icing colours separately into small bags. Cut off a tip in each bag and decorate the mushrooms with it