First make the syrup. To make a syrup out of a seedy fruit like an elderberry, add about 2 cups of the elderberries to a pot and pour 1 cup of water over them. Heat slowly until it simmers and the berries begin bursting. Keep the berries at a slow simmer and begin pouring in sugar. Start with 1/2 cup and stir it in well. Taste and add more if you want. Don’t go higher than 1 1/2 cups of sugar. Simmer for 5 minutes or so and then take off the heat.
Let the syrup cool a bit, then push it through a food mill with the finest setting, or a medium-meshed sieve. Elderberry seeds tend to be small, so you will need to account for that. Use a mesh that prevents them from getting through. Try to get as much pulp in as you can — it adds body to the syrup, and later the ice cream. Pour the syrup into a jar and chill it in the fridge. A syrup like this will last for weeks in the fridge, so you can make it far in advance.
When you want to make the ice cream, pour the cream and milk into a heavy pot and put it over a medium-high flame. Add the syrup. How much? I use 1 1/2 cups of elderberry syrup, but it will depend on your taste. Use as little as 1/2 cup, or as much as 2 cups. Just add and taste, and do it bit by bit, tasting as you go.
Bring the mixture to about 160°F, which is steaming but not simmering. Lower the heat a bit for now. Get a bowl and beat the egg yolks.
Now comes the tricky part: With a ladle in one hand and a whisk or fork in the other, slowly pour some hot cream into the egg yolks. Do this very slowly at first, whisking the eggs all the time with your other hand. Pour in another ladle, which can go in a little faster, then one more ladle, just to be sure. You are tempering your egg yolks, so they do not scramble in your cream mixture — this is making a custard. Pour the hot egg-cream mixture into the pot and whisk well.
Add the buttermilk – you can add more if you want, but use your taste as a guide. Whisk everything really well. Bring the mixture to 160°F, then take it off the heat. Again, this is not quite a simmer. Let it cool on the counter for 15 minutes or so, then add the liqueur. You use this both for flavor and to prevent the ice cream from setting up like a rock in the freezer. Put the mixture in the fridge to chill thoroughly, even overnight. Once it’s cold, pour into your ice cream maker and follow its directions.