In a large bowl using a hand-held electric mixer or in the bowl of a stand mixer fitted with the paddle attachment, cream the cream cheese, butter, and sugar on medium speed until light and fluffy, 2 minutes.
Add the flour and salt, and then mix just until a smooth dough forms.
Divide into 4 equal balls or squares. Wrap in plastic and chill for 1 hour or up to overnight.
Position a rack in the center of the oven, and heat the oven to 375 F.
Line 2 large rimmed baking sheets with silicone baking mats or parchment paper lightly coated with cooking spray.
Remove one dough square from the refrigerator and place it on a lightly floured work surface. (If you’ve refrigerated the dough overnight, let it sit out at room temperature for several minutes so it is more malleable and easier to roll out). Roll each dough into an 8-inch square. Spread one-fourth of the salted dark chocolate filling (recipe below) on the dough, leaving a 1/2-inch edge at the top and bottom.
Slice the square in half. Starting at a long end, roll each half into a cylinder from the outside in, and then cut each rolled half into six evenly sized pieces.
In a small bowl, whisk the egg and milk.
Arrange 12 rugelach on one of the prepared baking sheets.
Brush with the egg wash, and bake until golden-brown, 15 to 20 minutes.
Repeat with the remaining dough and filling.