Combine berries and nectar in a blender; process until smooth (about 1 minute). Add buttermilk; process until well blended. Add liqueur; pulse to mix. Add juice, if desired. Chill mixture 1 hour. Pour into freezer can of an ice-cream freezer; freeze according to manufacturer's instructions.
Pros...really low fat and easy. Cons...not very creamy, so this is more like a sorbet (fruit ice), and even with the liquor it freezes very hard so you have to chip it out to serve after re-freezing. If you want a very low fat frozen dessert, this is a reasonable choice. However, I found the recipe on this site for lemon buttermilk ice cream (also low fat, but not this low) wonderful, and in most situations I'd do that one over this one for a fruity frozen dessert. I wouldn't serve this one for guests, but I would serve the lemon buttermilk.
So yummy! Some tips: Whole buttermilk is hard to find. Buttermilk is so easy to make, so make your own. Take 3 parts whole milk, mix with 1 part buttermilk (any commercial brand with LIVE cultures - I used Friendship) Mix gently in a clean glass jar and loosen cover. Leave in a warm place for 12 hours and voila, buttermilk. It will look a little different than the store bought (thicker and not as smooth, but it is great). Follow the sherbet recipe. I didn't have raspberry liquor - I used 3 T rum and a Tablespoon of raspberry jam to heighten the berry flavor. The lemon juice made it a bit too tart so I had to add a Tablespoon of sugar to tone down the tartness. Also, for easy of measuring the berries,measure by displacement. Put the buttermilk in a 4 cup measure and then add whole strawberries (I used frozen organic) until the level rises by two cups. Then add the rest of the ingredients and blend the whole thing with an immersion blender.