Process raspberries in a food processor or blender until smooth, stopping to scrape down sides. Press raspberry puree through a fine wire-mesh strainer into a large bowl, discarding solids. Add sugar, buttermilk, and 1 teaspoon vanilla extract to bowl, and stir until well blended.
Pour raspberry mixture into freezer container of a 4-quart ice-cream maker, and freeze according to manufacturer's instructions. Garnish each serving, if desired.
*1 (14-ounce) package frozen raspberries, thawed, may be substituted for fresh raspberries.
Blueberry-Buttermilk Sherbet: Substitute an equal amount of fresh or frozen blueberries for raspberries. Proceed with recipe as directed. Garnish with fresh blueberries and lemon rind strips, if desired.
Blackberry-Buttermilk Sherbet: Substitute an equal amount of fresh or frozen blackberries for raspberries. Proceed with recipe as directed. Garnish with fresh blackberries and lemon rind strips, if desired.