Preheat your oven to 200ºc/180ºc Fan and line 3-4 baking trays with parchment paper.
Cream together the unsalted butter and caster sugar until light & fluffy, then gradually beat in the egg and vanilla bean extract so that the mixture is smooth.
Add in the Plain Flour and beat again till the dough is combined.
Roll the mixture out on a lightly floured work surface to the thickness of two £1 coins.
Cut them out in the various shapes that you want - I used 6cm size cutters but in various different shapes so that I had a mixture!
Place them onto the lined baking trays a couple of centimetres apart and bake in the oven for 8-10 minutes until they start to go golden around the edges.
Leave to cool on a wire rack.
Decoration
Add the egg whites to a large bowl, and add the icing sugar
Whisk together until smooth and thick. I start with the 400g, and add a bit more as I mix, to get the thick texture I am after
Colour the icing however you want - I did a variety of colours in separate bowls
Pipe this round the edge of the cookies and fill in the middle however you want - you can pipe around the edge with the firmer icing, and then slacken the rest with a little water to soften
Once they're all done, leave the icing to harden and dry, for at least 30 minutes
Notes
These will last 4-5 days in an airtight container, or you can freeze them for up to 3 months.