In the bowl of a stand mixer using a paddle attachment, cream the butter, cream cheese, vanilla, egg yolk and powdered sugar until light and fluffy, about 2 minutes. 

Add the salt and 3 cups of the flour, then mix on low until a smooth dough forms. Wrap the dough in plastic and refrigerate for 1-2 hours.

Combine all of the jam ingredients in a large saucepan over medium heat. Stir occasionally for the first 5 minutes and cook for a total of 25-30 minutes. At first, the mixture will look like it doesn't have enough liquid, but become juicy as the fruit releases water.

Around the 15 minute mark, don't abandon the jam! It will need to be stirred regularly, using a rubber spatula to scrape any fruit bits stuck to the side.

Cook the jam down to a thicker consistency than a typical jam, more like the texture of a paste. If the jam is too loose, it will spill out the sides of the rugelach, burn and make a mess.

Be careful to stir with caution as the jam will be boiling vigorously from the middle of the pot. Once paste-like consistency is reached, transfer to a jar to cool.

The yield should be 2 cups. Once the jam is cooled, stick a spoon in the jam. It should be spreadable with a spatula but too thick to flow off the spoon.

Preheat the oven to 375°. 

Divide the chilled dough into 2 equal parts.

Working with one piece at a time, roll a piece of dough between two pieces of parchment.

Use the remaining flour to keep the dough from sticking. Roll the dough to be 1/8-inch thick using a rolling pin. Aim to create a round shape, about 12-13 inches wide.

Peel off the top layer of parchment and spread half of the cooled strawberry rhubarb jam, about 1 cup, on top, leaving about 1 inch of the dough bare along the edge. Using a small offset spatula makes this task easy. 

Then, using a pizza cutter or knife, cut the dough into 12 even pieces, kind of like slicing a pizza. Gather the wide underside of the rugelach triangle and pull it out of the "pizza."

Fold over the wide edge and roll the triangle away from you. 

Continue rolling until the tip of the triangle point flips over the final edge, similar to rolling up crescent rolls. Place on a parchment lined sheet pan, 2-inch apart.

Repeat with the remaining dough.

In a small bowl whisk and combine the egg white with water. Brush all the cookies with the egg white to coat and sprinkle with sugar in the raw or sanding sugar. 

Bake until golden brown, 18 to 20 minutes. Let them cool completely before snacking.

Rugelach dough freezes well and can sit in the fridge up to 5 days. These cookies can be made ahead and are a great activity to do as a family. My pizza cutter has a fancy serrated side that creates a fanciful edge. Whatever utensil you use, wipe the blade between cuttings to keep the edges from tearing. Traditionally rugelach is a smaller, two bite sized cookie, but feel free to play with the size of the wedge slices. The bigger the slices the smaller the yield, but the larger the portion. With one test batch we cut the dough round into 8ths and the rugalach was about the size of a croissant, a perfectly sized pastry to enjoy with your morning cup of tea.