1) Place very clean glass or metal bowl and metal whisk in freezer just until cold, about 5 mins.
2) When everything is cold, place double cream in bowl and begin to whisk until it froths up and starts to thicken, about 1 min. Whisk vigorously until soft peaks form (the cream will hold a point when the whisk is lifted but will fall back into the bowl almost immediately), about 1 min.
3) Add the liqueur and whisk until stiff peaks just form (the cream stays put on the whisk when it is lifted from the bowl), about 1 min. Store covered in refrigerator until ready to use.