For the biscuits, cream the butter and sugar together until smooth. Beat in the egg and vanilla extract. Sift in the flour, baking powder and salt and mix until a smooth dough forms. Cover and chill for 60 minutes.
Preheat the oven to 200°C.
Roll out the dough to 1 cm thick on a lightly floured surface. Cut into rounds with a diameter of 5-7 cm roughly and transfer to a lined baking tray.
Make a thumb imprint in the middle of each cookie. Bake for 6-8 minutes until golden brown. Allow to cool on the tray before removing to a wire rack to cool completely.
Make the icing as the biscuits are cooling; beat the icing sugar with enough boiling water until you have a smooth, thick icing. Divide the icing into smaller pots and add a different food colouring to each.
Use a wet, warmed pastry brush to paint the icing onto the depressed centres of the biscuits. Leave the icing to set before serving.
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