In a large saucepan over medium heat, melt the butter. Add the onion and sauté until translucent, 2 - 3 minutes. Add the carrots, stock and mint sprigs. Cover and simmer over low heat until the carrots are tender, 25 - 30 minutes. Remove the mint sprigs and discard.

Place a colander over a bowl and pour the contents of the pan into the colander. Reserve the cooking liquid. Put the vegetables in a food processor fitted with the metal blade and purée until smooth. Return the purée and the reserved liquid to the saucepan. Over medium-low heat, stir in the milk and cream. Season to taste with salt and a little paprika and heat almost to a boil; do not allow the soup to boil.

Serve with a dollop of sour cream and a sprinkling of chopped mint on each serving.