Thaw frozen raspberries. Add to food processor or blender and puree until smooth. Strain puree into bowl. You will need to stir it around in the strainer and work it through. You should end up with about 2 cups of puree.
Add simple syrup, buttermilk, half & half, and vanilla to the raspberry puree. Chill in refrigerator several hours or overnight.
Pour raspberry mixture into ice cream maker and churn according to manufacturers directions. My machine took about 20 minutes.
Eat as soft serve or pour into container and freeze until firm.