Combine flour, sugar, and salt in the basin of a food processor² (see note if you don't have a food processor) and pulse to combine.
Sprinkle butter and cream cheese pieces over the top, add sour cream, and and pulse just into butter and cream cheese are cut into the dough. The mixture should still be quite crumbly and you will have chocolate chip-sized pieces of butter and cream cheese remaining. This is normal, don’t over-work your dough (see post or video for visual, if needed)!
Transfer dough to a clean surface and work the crumbs together gently (don’t over-handle or the warmth of your hands will start to melt the butter) to form a log.
Divide into 4 equal portions, flatten each into a disk that’s ½-1” thick. Wrap in plastic wrap and refrigerate for at least one hour and up to 5 days.
When dough has chilled, preheat oven to 350F (175C) and line a baking sheet with parchment paper. Set aside and prepare your chocolate filling
Chop or break chocolate bar into small pieces and heat in the microwave in 20-second intervals, stirring in between, until completely melted.
Add brown sugar, ground cinnamon, and salt and stir until combined. Mixture will appear grainy, this is normal (the "grain" from the sugar will disappear when the rugelach bakes) Set aside.
Working with one disk at a time, remove one round of dough from the fridge and transfer to a clean, lightly floured surface. Roll into a 10” circle and immediately spread ¼ of the chocolate mixture in a thin layer over the circle.
Use a pizza cutter or knife to cut the dough into 16 even wedges (I cut mine just like I’m cutting a pizza).
Starting from the larger side, roll one wedge over itself to form the rugelach and transfer to prepared baking sheet. Work quickly to roll each of the wedges as the chocolate will begin to harden fast and will make it difficult for you. However, even if the chocolate begins to solidify it should have been spread thinly enough that it should easily break as you roll and the rugelach should still look nice!
Space cookies at least 2” apart on baking sheet.
Prepare egg wash by whisking together egg with one teaspoon of water. Use a pastry brush to gently brush each cookie then sprinkle with coarse sugar or nonpareils.
Transfer to 350F (175C) preheated oven and bake in the center rack for 25 minutes or until golden brown.
Allow to cool completely on baking sheet before enjoying.