Preheat the oven to 180ºC/350ºF/gas 4. Line two large baking trays with greaseproof paper, then set aside.
Melt the butter in a pan over a low heat. Combine the sugars in a large bowl, pour in the melted butter and stir to combine. Beat the egg in a separate bowl, then stir into the mixture with the buttermilk. Sieve in the flour and bicarbonate of soda and fold through, then stir in the remaining ingredients.
Place heaped tablespoons of the mixture onto the prepared tray, flattening them slightly with your fingers and making sure you leave a 4cm gap between each. Place in the hot oven for around 14 minutes, or until golden and crisp.
Leave to cool for 5 minutes before transferring to a wire rack to cool completely. Enjoy with a glass of cold milk or a cup of steaming hot tea.