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To make the dough: Weigh your flour; or measure it by gently spooning it into a cup, then sweeping off any excess.
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In a large bowl or the bowl of a stand mixer fitted with the dough hook, combine all dough ingredients in the order listed, mixing until everything is moistened, 30 to 60 seconds. Cover the bowl and let the dough rest for 20 minutes. Then knead the dough until it's soft and smooth, about 2 minutes.
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Place the dough in a lightly greased bowl, cover, and allow to rise for about 1 1/2 to 2 hours, until puffy.
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To make the filling: In a small bowl, combine the brown sugar, cinnamon, and flour until well combined. Stir in the melted butter and water until evenly incorporated. Set aside.
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To make the topping: In a small bowl, combine the flour, brown sugar, cinnamon, and salt until well combined. Using your fingertips or the tines of a fork, work in the butter until coarse crumbs form. Set aside.
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Roll or pat the dough into a 9" x 18" rectangle about 1/4"-thick. If the dough snaps back, cover it and let it rest for 10 minutes to relax the gluten, then continue stretching until you reach the full size.
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Spread the filling evenly over the dough, leaving a 1" border bare along the edges. Scatter the nuts, if using, evenly on top of the filling and press them lightly to adhere.
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Starting with a short end, roll the dough gently into a log, sealing the seam and ends.
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Using a pair of scissors or a sharp knife, cut the log in half lengthwise to make two pieces of dough, each about 10" long.
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With the cut sides facing up, make an “X” with the two pieces of dough. Twist the two pieces together, then pinch the ends to seal. Gently tuck the ends under the loaf to hide the pinched area. Place the twisted loaf into a lightly greased 9" x 4" Extra Tall Loaf Pan or 9" x 5" loaf pan.
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Brush the loaf with the egg wash and sprinkle the crumb topping over the top.
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Cover the loaf and let it rise at room temperature until the crest of the loaf is about 3/4" to 1" below the rim of the 9" x 4" Extra Tall Loaf Pan, or 1" above the 9" x 5" pan, about 1 1/2 to 2 hours, depending on the temperature of your kitchen. Toward the end of the rising time, preheat the oven 350°F with a rack in the center.
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Bake the babka for 40 to 50 minutes, tenting with aluminum foil during the final 15 to 20 minutes of baking; the loaf should be a deep golden brown, and a digital thermometer inserted into the center should register about 195°F.
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Remove the Cinnamon Babka from the oven, and immediately loosen the edges with a heatproof spatula or table knife. Let it cool for 10 minutes, then turn it out of the pan onto a rack to cool completely.
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Slice the Cinnamon Babka and serve it at room temperature, or rewarm individual slices briefly in a toaster, if desired.
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Storage information: Store leftover Cinnamon Babka, well wrapped, at room temperature for several days; freeze for longer storage.
