Preheat oven to 150°CCream the butter and caster sugar together.
Beat in the egg and vanilla until well combined.Sieve the flour and add to the mixture. Flatten slightly, wrap in cling film and place in the fridge for 30 minutes. Sprinkle a little flour on to a work surface, roll out the cookie dough to an even thickness (approx 1/2 inch).
Cut out shapes with a Shamrock cookie cutter and carefully put them on a lined baking tray. Return to the fridge for another 30 minutes. Bake at 150°C for 20-25 minutes until the cookies are golden brown. Allow to cool on the tray for 10 minutes, then transfer onto a wire rack. Allow to cool completely.
To decorate the cookies with Siúcra Instant Royal Icing
Sieve the Siúcra Royal Icing Sugar into a bowl and follow the directions on the packet to make it. Divide the royal icing between two bowls. Colour with the green food colouring. Fill a small piping bag with the coloured icing and pipe around the edge of the cookie and inside of the cookie.
To decorate the cookies with Siúcra Roll Out Icing
Dust a work surface with Siúcra Icing Sugar. Lightly knead the Siúcra Roll Out Icing Sugar for 2 minutes. Sprinkle a little Siúcra Icing Sugar on a work surface and roll out the icing to 1/4 inch thick. Use the same shape cookie cutter as used for the cookies and cut out the icing. Spread a thin layer of Siúcra Instant Royal Icing onto the cookies then stick the roll out icing on top. The Siúcra Roll Out Icing can also be coloured with the green food colouring.