Dough:

In a food processor pulse flour, butter, egg yolk, sour cream, salt, and sugar until a coarse meal is formed. With machine running, add water and vinegar, a bit at a time, until dough pulls together.

Turn dough out onto counter and knead a few turns to combine. Flatten into a disk and wrap in plastic or parchment paper and refrigerate for at least 1 hour or for up to 2 days.

Assembly:

Preheat oven to 350°F. Line 2 baking sheets with parchment paper.

Roll dough out to quarter-inch thickness. Cut dough into 3-inch circles. Spoon sauce into center.

Sprinkle cheeses over sauce.

Brush edges with egg wash and fold over into a triangle, seal tightly, be sure corners are firmly pressed together. Place hamantaschen on prepared pan and leave about 2 inches between for dough to expand.

Bake at 350°F for 30 minutes, or until golden brown.