Preheat the oven to 180°C (160°C fan/350°F/gas mark 4). Add all the dry ingredients to a bowl and mix to combine. Add the cold butter and rub it into the flour with your fingertips. Keep going until you get a rough, clumpy mixture. Chill the mixture in the refrigerator for 15 minutes. Pour the mixture onto a baking sheet and bake for 15–20 minutes until golden, turning it once halfway through. Remove from the oven and let it cool completely – it will firm up as it cools. Store in an airtight container for up to 1 week.