Directions

1. If desired, prepare frozen lemon cups for serving: Cut off lemon tops and a thin slice from the bottoms (so the "cup" sits flat). Discard tops and bottoms. Squeeze the juice out of the lemons (use 6 tablespoons of the juice to make the frozen yogurt or freeze for another use). Using a paring knife, remove the flesh and seeds from each lemon, leaving a thin cup. Freeze the cups for at least 1 hour or until ready to use.
2. Warm sugar and water in a small saucepan over medium heat until the sugar dissolves. Transfer to a medium bowl and let cool, stirring, for 5 minutes. Add yogurt, buttermilk and lemon juice; whisk until smooth. Cover and refrigerate until very cold, at least 3 hours or overnight.
3. Pour the mixture into the canister of an ice cream maker and process according to manufacturer's instructions. Pipe into the frozen lemon cups, if using, or leave in the canister; freeze for 1 hour to firm up before serving. Transfer to an airtight container to freeze for up to 1 week.