In a small bowl, mix together the softened butter and miso paste until fully combined.
Add mirin, black pepper, and finely chopped chives, then mix again until smooth.
Spoon the butter onto a piece of baking paper or cling wrap, roll into a log, and twist the ends to seal.
Chill in the fridge for up to 2 weeks or freeze for up to 3 months.
Tip: Use a fork, spoon or spatula to mash and fold—miso can be stubborn to incorporate.