Preheat the oven to 180°C (350°F) (gas 4).
Beat butter, caster sugar, and coffee together until very light and fluffy, about 2 minutes.
Beat in the egg.
Now sift the flour, soda, and salt into a separate bowl.
Then fold a third of the flour into the creamed mixture.
Follow by adding a third of the buttermilk.
Repeat twice more.
Finally fold in the chocolate chips until evenly distributed.
Divide the mixture between the paper cases and bake for 25–30 minuets, until well risen and firm to the touch.
Remove from the oven and sprinkle demerara sugar over the tops.
Leave for two minutes and then transfer to a wire rack to cool.