Note: Chill for 2 hours, 30 minutes.
250g butter, at room temperature
1 cup caster sugar
1 teaspoon vanilla extract
1 cage-free egg
3 cups plain flour
1 1/2 teaspoons baking powder
2 cage-free egg whites
3 cups pure icing sugar, sifted
2-3 teaspoons warm water
assorted gel food colourings, for decorating
Preheat oven to 200C (180C fan forced). Line 2 large flat baking trays with baking paper. Cream butter and sugar together in a bowl, using electric beaters, until light and fluffy. Add the vanilla and egg and beat to combine.
Sift flour and baking powder together and add to mixture, 1 cup at a time, beating until just combined, after each addition. Divide dough into 2 portions. Chill for 10 min.
Roll out each dough portion on a lightly floured surface until 4-5mm in thickness. Cut into 8cm x 12cm rectangle shapes using a ruler and knife. Transfer to prepared trays. Re-roll dough and cut out as many rectangles as possible until all dough has been used. Bake cookies for 9-11 min or until lightly golden. Cool on trays for 5 min before transferring to a wire cooling racks to cool completely.
Beat egg whites in a bowl, using electric beaters, until frothy. Add the icing sugar, 1 cup at a time, beating after each addition until smooth. Add enough of the warm water, a few drops at a time, until it reaches a spreading, but slightly runnier consistency. Transfer to a shallow large dish.
Place a large wire cooling rack onto a flat work surface with a sheet of baking paper underneath. Working with one cookie at a time, dip one side into the icing to coat evenly. Allow excess icing to drip off then quickly turn upright and place onto wire rack to set. Repeat with all cookies until coated. Allow to dry for 3-4 hr or overnight if time permits, until icing has set firm.
To decorate, place 1-2 drops of your chosen gel colours into a small paint pallet. Add 1-2 drops of water to each colour and mix to combine. Using a small paint brush, dip into the colour of choice then blot on paper towel to remove excess. Let the kids paint as desired washing paint brush well between colour changes. Allow the paint on the canvas cookies to dry before eating.