Prepare two baking trays lined with baking paper.
Cream the butter and sugar together until pale and fluffy. Beat in the egg and vanilla.
In another bowl, combine the flour, baking powder and salt. Add the dry ingredients to the butter and eggs, and mix it just starts to form a dough.
Shape into a thin disk and wrap in clingfilm. Allow to rest in the fridge for at least 1 hour.
When you are ready to make the cookies, preheat your oven to 180ºC/fan 160°C.
Sprinkle your work bench flour, place the disc of dough on it, and sprinkle with a light dusting of flour. Roll out to a thickness of about 5mm. Cut into shapes using cutters. If you find that the cutter sticks to the dough, dipping the cutter into flour as you go usually helps.
Place the biscuits a little apart on the baking sheets.
Bake for 8–12 minutes, or until lightly golden around the edges. Place on a write rack to cool.
When the cookies are cooled, get on with the icing. Place the icing sugar in a bowl, and add just-boiled water - a teaspoon at a time - until it forms a thick icing.
Decorate the cookies. I find that it's easier to pipe a thin line of icing around the edge of the cookies, then fill in. At home, I'll usually pipe the outline and then let Sienna fill the cookies in and add sprinkles.
Let the icing harden. When completely hard, transfer the cookies to an airtight container. They will keep for a week or two at room temperature.