Raw version: let the bars firm up in fridge or freezer for 15 - 20 minutes. (nutritionrefined.com)
Then, place mug in fridge or freezer until chilled. (greatist.com)
Or might you want to move in a spare fridge or freezer so that it functions as a utility - cum - pantry space where you can also do some food prep. (idealhome.co.uk)