In a large mixing bowl, combine the softened salted butter and cream cheese. Use a hand mixer or a spatula to blend them until smooth and creamy. Gradually add the all-purpose flour and 1/2 cup of sugar, mixing until the dough comes together.
Divide the dough into four equal parts. Shape each portion into a disk and wrap them in plastic wrap. Chill in the refrigerator for at least an hour or until firm.
While the dough is chilling, prepare the filling. In a medium bowl, combine the frozen sweet cherries, 1/4 cup of sugar, cornstarch, and chopped walnuts. Gently toss to coat the cherries and set aside.
Once the dough is firm, preheat your oven to 350°F (175°C). On a lightly floured work surface, take one disk of dough and roll it out into a circle about 1/8 inch thick.
Spread a portion of the cherry filling evenly over the rolled-out dough, leaving a small border around the edges.
Using a sharp knife or pizza cutter, slice the dough into 8-12 wedges, depending on how large you want your rugelach. Starting from the wide end of each wedge, roll it up toward the point, forming a crescent shape.
Place the rolled rugelach on a baking sheet lined with parchment paper. Beat the egg with milk or water and brush the mixture over the tops of the rugelach. Sprinkle with additional sugar or sparkling sugar for a delightful finish. Bake for 20-25 minutes or until golden brown.
Remove the rugelach from the oven and let them cool on the baking sheet for a few minutes before transferring to a wire rack.
