In a bowl, mix flour, butter, icing sugar, egg yolk, baking powder and salt. Knead into a dough and chill one hour in the fridge.
Preheat oven to 180 degrees C and line baking trays with baking paper.
Roll dough out to 5mm thick and cut out cookies with oval or egg-shaped cutters. Place on baking trays and bake 10-15 minutes or until lightly tan. Let cool on a rack.
Make the icing: in a bowl, mix icing sugar with cream of tartar. Mix in the egg white, then use an electric beater to combine until smooth. If icing is too dry, add water teaspoon by teaspoon until smooth and thick.
Divide the icing into three portions and colour one portion pink and another yellow. Using piping bags with small nozzles decorate the cookies to look like Easter eggs. Add sprinkles as desired.