Savory Latke Babka

Ingredients

• 6 tablespoons whole milk, at room temperature
• 1 1/2 teaspoons instant yeast (from 1 [1/4-ounce] envelope)
• 1 large egg
• 1 large egg yolk
• 2 1/3 cups all-purpose flour (about 10 ounces), plus more as needed and for surface
• 4 tablespoons unsalted butter, softened
• 3 tablespoons sour cream
• 1 1/2 tablespoons granulated sugar
• 1 3/4 teaspoons kosher salt
• Cooking spray
• 1 large egg white, beaten
• Flaky sea salt, to garnish
• 5 tablespoons olive oil, divided, plus more as needed
• 1 yellow onion, chopped (about 1 1/4 cups)
• 1 large russet potato, scrubbed
• 1/2 cup sour cream
• 1/4 cup breadcrumbs
• 1/2 teaspoon kosher salt, plus more to taste
• Freshly cracked black pepper, to taste

Steps

1. Make the dough: Whisk together milk and yeast in the bowl of a stand mixer fitted with dough hook attachment. Let stand until yeast is dissolved, about 3 minutes (it will not be foamy).
2. Add egg and egg yolk, flour, butter, sour cream, sugar, and salt. Beat on low speed until incorporated, about 1 minute. Increase speed to medium, beat until dough pulls away from sides of bowl and is smooth, soft, and very elastic, 12 to 15 minutes. If dough is very sticky and has not pulled from sides of bowl after 10 minutes, add up to 2 tablespoons additional flour, 1 tablespoon at a time, allowing it to incorporate fully before adding more, just until the dough comes together. Cover bowl with plastic wrap and set aside at room temperature until doubled in size, 1 hour to 1 hour, 30 minutes.
3. While dough proofs, make the filling: Heat 2 tablespoons oil in a large skillet over medium-high. Add onions; cook, stirring often, until deep golden brown, 8 to 10 minutes. Transfer onions to a medium heatproof bowl and remove skillet from heat. Using largest holes of a box grater, shred potato. Squeeze shredded potato dry using a clean kitchen towel.
4. Heat remaining 3 tablespoons oil in same skillet over medium-high until oil begins to shimmer, about 3 minutes. Add shredded potatoes and stir to coat evenly with oil, about 1 minute. Spread potatoes in an even layer in bottom of skillet; cook, undisturbed, until potatoes begin to brown and become crisp on the bottom, 3 to 4 minutes. Using a spatula, stir potatoes and spread in an even layer; repeat browning and stirring process, stirring every 3 to 4 minutes, until potatoes are golden brown and crispy all over, adding additional oil as needed if skillet seems to be getting dry, 10 to 12 minutes total. Transfer browned potatoes to bowl with onions. Stir, breaking up any large clusters of potatoes, until evenly distributed. Set aside to cool to room temperature, 20 to 30 minutes. Once cool, stir in sour cream, breadcrumbs, and salt. Taste, add pepper and additional salt, as desired.
5. Lightly coat a 9- x 5-inch loaf pan with cooking spray. Line bottom and long sides with parchment paper, leaving at least a 1-inch overhang on long sides; set aside.
6. When dough has doubled in size, gently deflate with hand, and turn out onto a lightly floured work surface. Roll dough into a 9- x 20-inch rectangle. Spread cooled potato filling in an even layer over dough, spreading out to edges; press filling firmly into dough. Beginning with one of the short sides, tightly roll dough to form a 9-inch-long log.
7. Using a sharp knife, cut log in half lengthwise to expose the filling, making two 9-inch strands. Cross dough strands to make a large X shape, then twist strands together to make a 9-inch spiral, keeping cut sides facing up. Transfer twisted dough to prepared pan, tucking edges under as needed. Cover loosely with plastic wrap and set aside in a warm place (75°F) until puffy and dough reaches top edge of pan, 30 to 45 minutes. While dough proofs, preheat oven to 350°F.
8. Gently brush top of the dough with beaten egg white and sprinkle with flaky salt, if desired. Bake the babka until deep golden brown and a thermometer inserted into center registers 195°F, 50 to 60 minutes. If necessary, loosely tent top with aluminum foil during last 15 minutes of baking to prevent overbrowning.
9. Let cool in pan on wire rack for 15 minutes. Using excess parchment as handles, remove babka from pan; discard parchment. Let cool slightly, about 45 minutes, before slicing.