Knead flour, sugar, vanilla sugar, egg and butter in flakes with the dough hooks of the hand mixer to a smooth dough. Wrap in foil and chill for 30 minutes. Roll out the dough thinly on a well floured work surface and cut out 15 boots (14 x 6 cm) or make a template and cut out. Put the boots on 2 baking trays lined with baking paper and bake in a preheated oven (electric range: 200 °C/ convection oven: 175 °C/ gas: level 3) for 10-12 minutes. Remove the boots from the oven, place them on a cake rack and let them cool down.
In the meantime, mix lemon juice and icing sugar. Spread the icing evenly on 6 bowls and colour with a few drops of food colouring green, red, pink, light blue and violet. Pour each colour into a freezer bag and cut off a small corner. Decorate boots differently and let them dry