In a stand mixer fitted with the whisk attachment (or in a large bowl with a hand-held mixer), whisk the cream, yogurt, and salt together on medium-high speed for about 2 minutes, or until soft peaks form. Fold the cranberry sauce into the cream a little at a time, being careful to leave streaks instead of stirring everything together. Divide the fool among 4 bowls (preferably glass so that you can see the swirls) and serve immediately.