Boil the carrots in salted water for 20-25 mins until tender. Drain in a colander and leave to steam-dry for 1-2 mins. Return to the pan with the butter and roughly mash if you would like a little texture in them, or blitz with a hand blender for a silky-smooth purée. Season well and stir in the tarragon. Will keep in the fridge for 3 days. Reheat
in the microwave for 2-3 mins, or in a pan.