In a bowl, whisk together 100g butter and 150g soft light brown sugar until pale and fluffy. Beat in the egg then stir in the flour, orange zest, chopped dates and walnuts, bicarbonate of soda and sea salt until combined. Chill for an hour.
Using an ice cream scoop, shape the dough into 12 balls and arrange on a lined baking tray. Cover and freeze until needed.
When you’re ready to bake, arrange the cookie balls on 2 greased and lined baking trays. Leave to rest at room temperature for 30 minutes until soft but still chilled. Meanwhile, preheat the oven to 180°C, gas mark 4.
Lightly press down on the balls. Bake for 10 minutes, then remove from the oven and leave to cool on the trays for 10 minutes. Transfer to a wire rack to cool completely.