To make the crust using a food processor: Place the flour and salt in the bowl of a food processor. Pulse briefly to combine. Cut the butter and cream cheese into chunks and add to the bowl along with the sour cream. Pulse just until the dough forms chunks, and you can squeeze it together., To make the dough using a mixer: Beat together the butter, cream cheese, sour cream, and salt until smooth. Add the flour, mixing to make a stiff dough., Divide the dough into three equal portions. Press each gently into a disk. Make the disks as round as possible, smoothing their edges; this will allow you to roll the disks into a perfectly round circle, making the resulting rugelach more attractive. Wrap the disks in plastic, and chill the dough for about 1 hour, until it's firm but not rock hard. Or chill longer (up to overnight), then warm for about 45 to 60 minutes at room temperature, until the dough softens enough to roll out without cracking., To make the filling: Process the sugar, walnuts, dried fruit, and cinnamon in a food processor or blender until finely chopped and well combined (but not pasty). Don't have a food processor? Simply stir together the filling ingredients; your filling will be chunky rather than smooth., Working with one piece of dough at a time, place it on a generously floured surface. Roll it into a 10" circle and brush it lightly with water. For a flavorful touch, brush the rolled-out rugelach dough with a thin layer of boiled cider, warmed apple or currant jelly, or puréed fruit preserves, instead of water., Use a small offset spatula to spread about 1/3 of the filling onto the round, going all the way to the edges and gently patting the filling to help anchor it to the dough., Using a pizza cutter, baker's bench knife, or sharp knife, divide the dough into 12 equal wedges. Roll each wedge up, beginning with the wide end and ending with the narrow end. Place the rolls point-side down on a baking sheet; lining the baking sheet with parchment will help with cleanup. Repeat with the remaining two pieces of dough., Brush the rugelach with milk or cream; and sprinkle with granulated or coarse sparkling sugar., Preheat the oven to 350°F. Refrigerate the rugelach while the oven is preheating., Bake the rugelach for 25 to 30 minutes, or until golden brown. Remove from the oven, and cool right on the pan. Serve warm or at room temperature., Store leftover rugelach in an airtight container at room temperature for several days. Freeze for longer storage.