Mix flour, baking powder, sugar and salt in a mixing bowl. Spread softened butter in flakes, lemon zest and egg on top. Work into a smooth dough with the dough hooks of the hand mixer. Wrap in cling film and chill for about 1 hour. Roll out the dough on a floured work surface to a thickness of approx. 4 mm. Cut out funny sea animals with various cutters and place them on 2 baking trays lined with baking paper.
Bake the trays one after the other in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for approx. 10 minutes. Take out and let cool down. Stir icing sugar, egg white and lemon juice until smooth. Divide the glaze into thirds, colour one part of it with a few drops of blue food colouring and one part with a few drops of green food colouring. Fill the icing into the piping bags, cut off a small corner of each and decorate the animals with it