To make the biscuits, beat the butter with the icing sugar until pale. Beat in the zest and vanilla.
Sift in the dry ingredients all together and stir until just combined. Roll large teaspoons of the mixture into balls. Place on non-stick paper on a flat baking tray, giving them room to expand.
Bake in a 180 degrees C oven for 12-15 minutes or until just lightly browning on the bottom. Cool.
To make the icing, beat the butter with icing sugar until smooth. Beat in the zest and juice. Spread onto half the biscuits and sandwich with the other halves while the icing is still soft. Store in an airtight container.