In a large bowl, mix together the flour, baking powder and salt. Set aside.
Place softened butter in a bowl of an electric mixer and beat it on medium-high speed until pale and fluffy, about 2 minutes. Add sugar and cream it together for another 2 minutes. Add egg and vanilla and beat on high speed until well combined. Scrape the sides and the bottom of the bowl to make sure you combine it all together.
Add the flour mix into the butter and sugar mixture and mix on low speed until combined. If the dough feels too sticky to roll, add one more tbsp of flour. Divide dough into two parts and roll each part on a lightly floured silicone mat or baking paper to about £1 coin thickness. (about 1/4' ). Transfer the rolled dough onto baking tray and place in the fridge for minimum of 2 hours (if chilling overnight, cover the dough with some cling film or re-usable wrap).
Preheat the oven to 170 C (fan). Line the baking trays with baking parchment. Remove one of the pre- rolled dough pieces from the fridge. Using cookie cutters, cut the desired shapes out of the dough and transfer into baking trays. Re-roll any scraps and cut the cookies again, until you have used all of the dough.
Bake in the middle shelf of the preheated oven for about 8-10 minutes until the edges are lightly golden. Rotate the baking tray with cookies half way through baking time. Take out of the oven and allow them to cool for about 5 minutes on the baking trays, before transferring onto a cooling rack to cool completely.
To make the icing
Place the egg whites and cream of tartar in a bowl of a standing mixer fitted with whisk attachment (or an electric mixer fitted with beaters) and mix until soft peaks form (about 2 minutes).
Sift in the icing sugar and beat on low speed for another 2 minutes. With the mixer running on low, keep adding some water, 1 tbsp at a time, until the icing reached the desired consistency. When lifting the whisk up from the icing, the icing should drizzle down and smooth out within 10-12 seconds ('draw' a line with the dripping icing, if it disappears after 10-12 seconds, it is ready). If you want your icing to be thinner, keep adding more water, if you want it to be thicker, add more icing sugar.
If using food colouring, divide the icing into separate bowls and mix with gel food colouring. Keep checking the consistency, as adding the food colouring may change the consistency of your icing. Transfer the icing into piping bags. Snip the very end of each piping bag and decorate the cookies.
The number of cookies will depend on the size of your cookie cutters
You can fit piping bags with small piping nozzles, but it is not necessary
If not using piping nozzles, snip each piping bag at the very end of it. You want a tiny opening that will create a thin flow
Make sure the cookies have cooled down completely before decorating them with icing
Allow the icing to dry before adding more detail made out of icing again
If you have royal icing sugar, you can use that instead to make your icing (follow the recipe as per instructions on the pack)