Cream together the butter and brown sugar until light and well combined. Blend in the molasses (or Black Treacle if you can't find Molasses) and the egg.
Dissolve the bicarbonate of soda in the boiling water and add to the mixture then stir through the vanilla extract.
Meanwhile sift together the remaining dry ingredients in a separate bowl then add gradually to the wet mixture. Continue mixing to combine well and it will come together into a soft dough. Set aside the mixture to cool for a couple of hours.
When ready to bake roll out the dough on a floured surface to 1/2cm thickness. Cut to shape using a cookie cutter.
Bake in an oven at 140CFan for 15 minutes. They will puff up in the oven and settle again once rested. Set on a wire rack and allow to cool.
These cookies are best eaten the day after they are baked as the flavour develops.