With an electric mixer, beat butter until light and fluffy. Gradually beat in the brown sugar, egg and vanilla.
In a separate bowl, combine flour, bicarb soda and salt. Beat into the creamed mixture until just incorporated.
Roll the dough out to 5mm thickness between two sheets of waxed paper. Remove the paper and cut with cookie cutters to desired shapes. Place 3cm apart on ungreased baking trays. Sprinkle with the coloured sugars to decorate.