Combine the cut up plums and water in a heavy, non-reactive 5-quart saucepan. Bring it to a boil then reduce the heat to a steady simmer, cover, and cook 20 minutes.
Pour the fruit and water through a strainer with a bowl to catch the juices, letting it drain 15 minutes. Measure the drained liquid. Measure out the same amount of sugar. Return the juices to the saucepan and bring them to a boil. Add 1 tablespoon lemon juice and begin adding the sugar 1/2 cup at a time, allowing the mixture to return to a boil before adding the next 1/2 cup sugar. Continue cooking until the liquid reaches its jell point (generally 8 degrees above the boiling point measured on a thermometer). This takes about 5 or 10 minutes.
Turn off the heat and stir in the plum pieces, letting them steep together for 15 minutes.
Return the preserves to a boil. Stir in the remaining 1/2 tablespoon lemon juice and 1/2 cup more sugar and boil for 10 minutes stirring frequently and the temperature rises to 215 degrees F. Turn off the heat and skim off any foam. You may allow it to cool then chill it or to can it, pour into jars, wipe the rims, place the lids on, and invert the jars for a quick vacuum seal or process in a boiling water bath submerged 1-inch for 10 minutes.
In a large bowl, combine the flour and salt. Add the butter and, using your fingertips, pinch the ingredients together into a sandy, crumbly mixture.
In a bowl, stir together the sour cream and egg yolk until very smooth. Add to the flour mixture and mix until the dough comes together. The dough may feel a little dry. Form into a disk, wrap it in plastic wrap, and chill at least 4 hours or overnight. On a work surface thickly dusted with powdered sugar, roll out the pastry 1/8-inch thick. Cut out 4-inch disks and place them on a sheet try in case you need to chill them. Brush milk around the edge of each one. Place a tablespoon of filling in the center of each disk, fold it over, and seal the edges with a fork. Chill 30 minutes.
Preheat oven to 375 degrees F. Bake the pastries until light golden brown, 20 to 25 minutes. Let cool on the pan on a wire rack.