Line a 9x13 pan with parchment paper, preheat oven to 325F
Beat the butter until smooth.
Add the icing sugar, and beat until fully combined.
Slowly beat in the flour.
Form the dough into a smooth ball. If you can't, lightly knead the dough.
Gently press into the pan, use a small rolling pin, but don't press hard. We want it smooth, but pressing too hard will make the cookies too firm.
Bake 20 minutes, until edges turn lightly brown.
Remove from oven.
Quickly cut into the shapes you want, lightly prick with a fork.
Return the baking sheet to the oven for 8-10 minutes, until a light golden colour.
Turn off the oven, and prop the door ajar with a ball of foil.
Let it cool for an hour, then transfer to a rack to cool completely.