For the dough, add the flour, sugar, and yeast to a stand mixer fitted with the dough hook. Mix on low speed for 1 minute. Add the eggs and water and mix on low speed for a few seconds, then increase the speed to medium and mix for 3 minutes, until the dough comes together. Add the salt and then start adding the butter, a few cubes at a time, mixing until it is incorporated into the dough. Continue mixing for about 10 minutes on medium speed, until the dough is completely smooth, elastic, and shiny. During the mixing, you will need to scrape down the sides of the bowl a few times and add a small amount of flour onto the sides so that all of the dough doesn't stick too much.
Transfer the dough to a large, lightly greased bowl. Cover with plastic wrap and refrigerate overnight.
Grease two 9- by 4-inch loaf pans with butter and line the bottom of each pan with a piece of parchment paper. Divide the dough in half and keep one half covered in the fridge.
On a lightly floured surface, roll out the dough into a rectangle measuring about 15- by 11-inches. Trim the sides to make them more or less even. Use an offset spatula to spread half the Nutella on the dough, leaving a 3⁄4-inch border all around. Sprinkle half of the halvah over the top.
Brush a little bit of water along the long end farthest away from you. Use both hands to roll up the dough into a long log. Press to seal the dampened end. Transfer the dough to a sheet pan, seam side down, and freeze for 15 minutes to make the next step a bit easier. Repeat the rolling, filling, and re-rolling with the other half of the dough and filling.
Working with one roll at a time, use a serrated knife to trim about 1/2 inch off both ends of the roll. Use the knife to gently saw the roll into half lengthwise. With the cut sides facing up, gently pinch one end of each half together, and then lift the right half over the left half. Repeat this process, but this time lift the left half over the right so that you're making a twist. Try to keep the cut sides facing outward. When you get to the end, gently squeeze the two halves together. Carefully lift the cake into one of the prepared loaf pans. Cover the pan with a damp tea towel and leave to rise in a warm place for 1 to 1 1/2 hours. The loaf will rise and get puffy but will not double in size. Repeat the process to make the second loaf.
Preheat the oven to 375° F. Remove the tea towels, place the cakes on the middle rack of the oven, and bake for about 30 to 40 minutes, until a skewer inserted in the center comes out clean.
While the cakes are in the oven, make the syrup. Combine the water and sugar in a saucepan, place over medium heat, and bring to a boil. Cook until the sugar dissolves, then remove the pan from the heat. As soon as the cakes come out of the oven, brush them liberally with the syrup. Use all of it. Let the loaves cool for about 30 minutes, then remove them from the pans to cool completely.
Yossy Arefi is a photographer and stylist with a passion for food. During her stint working in restaurant kitchens, Yossy started the blog Apt. 2B Baking Co. where, with her trusty Pentax film camera, she photographs and writes about seasonal desserts and preserves. She currently lives in Brooklyn but will always love her native city of Seattle. Follow her work at apt2bbakingco.blogspot.com & yossyarefi.com.