Preheat the oven to 180°C/gas mark 4. Beat together the butter and sugars using a handheld electric mixer for 5 minutes until light and fluffy. Beat in the egg and vanilla extract, then gently fold through the flour, pistachios and cranberries.
Line two wide baking sheets with greaseproof paper and place generous tablespoons of the mixture on them, keeping them well spaced apart. Flatten slightly with the back of a spoon, then bake for 15-18 minutes, until golden around the edges and still slightly soft in the middle. Transfer to a wire rack to cool.