First, remove the stems from the cherries. Then, place in a colander and rinse well under cool water. Gently rub with your hands, making sure there are no leftover leaves or dirt. Remove the cherry pits.
Add the water, sugar, and salt to a large pot. Bring to a boil and then reduce the heat to medium (if you want to add spices too, e.g. cinnamon stick, cloves, etc., add them now together with the sugar).
While the soup is coming to a boil, prepare the thickening mixture. Whisk the egg yolk with the sour cream, the flour and two tablespoons of cold milk. Set aside.
Once the water is boiling, add the pitted cherries to the pot and simmer on medium heat for 15-20 minutes. The cherries should soften, but still hold their shape. Make sure they do not become mushy.
Remove the pot from the heat (remove the cinnamon stick if you used one). Ladle a bit of the soup to the sour cream mixture to temper it. Do this 3 times, mixing well. Then pour the mixture slowly into the hot soup, stirring constantly. Add the butter.
Place the pot back on the heat. Simmer for 5-6 minutes, until the soup thickens a little bit. For a creamier texture, you can add some milk at this point.
Finally, taste and adjust the salt and sugar. If the soup tastes a bit bland, it's missing salt. So, keep adding a bit more salt, until you feel that the soup starts to be yummy. Then, adjust the sugar. Don't add too much.
Let the soup cool completely at room temperature. Refrigerate for at least 4 hours. Ideally overnight.
Garnish with the reserved cherries (optional) and serve.