Boon of Winter's Bite

You've fought against the elements and overcame the harshest of winter, defeating chilly foes and terrors alike; your soul touched by their essence. Once per long rest, as a bonus action, you may wreathe yourself in an aura of chilling energy. For the next minute, when you hit a creature with an attack roll, you deal an additional amount of cold damage equal to your Proficiency Bonus.

Additionally, once per long rest, as a reaction to taking cold damage, you may give yourself resistance to that instance of damage.