Caster sugar(for coating)
In a bowl, cream the butter and icing sugar.
Lightly whisk the egg. Once the sugar mix is light and fluffy, add in the egg, zest and vanilla.
Sift the bakers flour. Add it to the above mixture and mix well.
Make a log. Brush with egg wash and coat in caster sugar.
Slice 1 cm in thickness and bake at 180 degrees C for 10 minutes.