Beat together the butter, treacle, brown sugar and salt in a large mixing bowl. Add the egg, then beat through all of the remaining dry ingredients apart from the caster sugar. Chill the mixture in the fridge for 1 hr.

Heat the oven to 200C/180C fan/gas 4. Line two baking sheets with baking parchment. Roll the mixture into 20 even-sized balls (weighing for accuracy, if you like). Tip the caster sugar onto a small plate, then add each ball and roll around to coat. Space each ball out on the baking sheets. Bake for 9-10 mins until golden brown. Leave to cool completely on a wire rack.