Combine the flour, butter, icing sugar, egg yolk, baking powder and salt and knead into a dough. Chill 1 hour in the fridge.
Preheat oven to 180 degrees C and line baking trays with baking paper.
On a floured surface, roll out the dough to about 5mm thick and cut out biscuits with a rabbit-shaped cutter. Place on the baking trays and bake 10-15 minutes or until very lightly brown. Let cool on a cake rack.
To make the icing, combine the icing sugar and cream of tartar in a bowl. Add the egg white and stir with a spoon to moisten the sugar. Beat with an electric mixer until well combined - if too thick, add a little water.
Fill a piping bag with icing and ice the biscuits, starting with the outline. Use pink sprinkles for the noses and chocolate sprinkles for the eye.