Using a mixer, beat the butter on medium speed to loosen it, about 5 to 10 seconds. Add the cream cheese and combine at medium speed, about 10 to 15 seconds. Add the sugar, beat on medium to aerate, about 3 minutes. The dough should look like and fluffy. Scrape down the sides and bottom of the bowl.

Add the vanilla and both salts, mix briefly to incorporate. Scrape down the bowl again.

Add the flour, mix on low speed until the dough just comes together, about 30 seconds. It should still look a bit shaggy.

Bring the dough fully together by hand or with a bench scraper. Divide dough in half (each should way 14.5 ounces). Place each half on a separate piece of plastic wrap, pat the dough into a rectangle, wrap tightly and refrigerate at least 2 hours or up to 1 week.

In a large (6-qt), heavy pot, combine the cream, sugar and syrup and cook over medium heat for about 3 minutes, until the sugar has dissolved.

Add the chocolate and salt, bring to a boil. Immediately lower to a gentle simmer, and cook until the mixture breaks and oils begin separating from solids, about 40-45 minutes. Stir occasionally to avoid burning the bottom.

Whisk in the butter and vanilla (or use an immersion blender to get the sauce really, really smooth). Let cool.

Store in the refrigerator up to 6 months.

Freeze the toasted hazelnuts and chocolate pieces until thoroughly chilled, about 20 minutes.

Using a food processor, pulse to grind the nuts and chocolate together. Add the cocoa nubs, sugars, flour, and salts; continue pulsing until mixture is the texture of a coarse meal (should take less than 10 pulses).

Place a 13 by 18-inch (same dimensions as a half sheet pan) piece of parchment paper on the work surface. Dust with flour. Unwrap one of the dough halves and place on top of the dusted paper.

Roll the dough out until it forms a rectangle ending 1-inch from the edges of the parchment paper (12 by 17 inches). To prevent the dough from sticking, periodically dust the dough with flour, cover with another piece of parchment paper, and flip the dough (and paper) over. Peel off the top piece of parchment paper and continue rolling. Even out the edges with a bench scraper to straighten the rectangle. In the end, the dough should be just under ¼-inch thick.

Repeat steps 1 and 2 with the second half of dough.

Stack both rectangles of rolled out dough and refrigerate to chill, about 30 minutes.

Heat the oven to 30 F. Line several half sheet pans with parchment paper.**

Remove both sheets of dough from the fridge and peel off the parchment paper on top.***

Evenly spread half the hot fudge sauce in a thin layer across each dough rectangle. (No need to leave a border along the edges.)

For each dough sheet, sprinkle ½ cup of the streusel on top of the hot fudge layer.

Trim the edges of your dough if you are going for perfection. Using a dough cutter or pizza wheel, divide each sheet in half lengthwise. From each long strip, move crosswise to cut out flat-tipped triangles, with a base of 1.5 inches wide and a tip of ¼ inch wide. You should get 12 triangles per strip (48 total).

Separate a triangle from the rest of the dough (an offset spatula or bench scraper are perfect tools for this). Starting from the base, roll the triangle up like a croissant or crescent roll. Place it on the parchment papered-sheet pan, tip side up. Repeat roll-up process for remaining triangles, spacing them 2 inches apart on the prepared sheet pans (necessary for letting the hot fudge-streusel spread).

Brush the tops of each rolled cookie with egg white, and sprinkle with the remaining (~1 cup) streusel.

Bake only one pan at a time. Bake for 15 minutes, rotate, then bake another 5-10 minutes, until the hot fudge has oozed out and bubbled along the sides.

Let the cookies cool completely on the sheet pan (or let cool 5 minutes, transfer entire sheet of parchment paper with cookies on top to a cooling rack, and reuse your pan). Repeat baking process with remaining dough.

Rugelach will last 5 days in an airtight container at room temperature. Before adding the egg white wash, the rolled dough can be refrigerated for one week.