The outermost layer of the lemon, called the zest, is full of essential oils. Remove the zest by grating it with the fine rasps of a hand-held grater.
In a small bowl, combine sour cream, powdered sugar, lemon zest, lemon juice and vanilla. Whisk to mix evenly. Cover and chill for several hours.
Divide grapes equally among 6 stemmed dessert glasses or bowls. Add a dollop of sauce to each dish and top with 1/2 tablespoon of chopped walnuts. Serve immediately.
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