1. Bring a saucepan filled with 200ml of water to a gentle simmer over a medium heat. Add the juniper berries, cover with a lid and simmer for 10 – 20 minutes until the berries soften and begin to break down. Strain the water into a jug and set aside.
2. In a large mixing bowl, use a fork to whisk the blueberry juice with the gelatine then set aside. Put the egg white into a bowl of a stand mixer.
3. Warm the honey and fruit syrup with 130ml of juniper water in a saucepan over a medium heat. Bring to the boil until it reaches 125oC. Just before the juniper liquid reaches temperature, whisk the egg white to soft peaks.
4. Quickly but carefully add the blueberry and gelatine mixture to the juniper liquid, then slowly pour this onto the egg white with the mixer on. Whisk at full speed until the mixture has become light, glossy and doubled in volume. This will take about 8 minutes. Add the vanilla extract, whisk for another 2 minutes then stop.
5. Pour the mixture into a prepared baking pan and smooth out the top. Put in the fridge for at least 3 hours to set.
6. Lift the marshmallow from the pan using a cling-film/plastic wrap. Carefully grease a sharp knife with a light flavourless oil and use to cut into squares. Dust the cut marshmallows in arrowroot to coat and seal them so they are not too sticky to pick up.