2 sticks unsalted butter at room temperature
½ teaspoon flaky sea salt (or a heaping ½ teaspoon kosher salt)
8 ounces cream cheese at room temperature (preferably natural cream cheese without stabilizers)
1¾ cups all-purpose flour, sifted, plus extra for rolling
1⅔ cups black poppy seeds
¾ cup honey
⅓ cup water
1 teaspoon lemon zest plus 1½ tablespoons fresh lemon juice
1 teaspoon orange zest plus 1½ tablespoons fresh orange juice
¼ teaspoon flaky sea salt (or a heaping ¼ teaspoon kosher salt)
2 tablespoons granulated sugar
½ teaspoon ground cinnamon
Pinch freshly grated nutmeg
All-purpose flour, for dusting
1 large egg, lightly beaten
1. Make the dough: To the bowl of a stand mixer fitted with a paddle attachment, add the butter and salt. Add the cream cheese in large pieces and mix on medium-low speed until combined (if using natural cream cheese, the mixture will not be completely smooth).
2. Turn off the mixer, add the flour and reduce the speed to low, mixing until the dough is shaggy and has a few dry spots but starts to gather around the paddle into a ball, about 1 minute. Use a rubber spatula to transfer the dough to a large sheet of plastic wrap. Fold the plastic wrap over the dough and use the plastic wrap to help shape the dough into a 1-inch-thick rectangle. Wrap the dough tightly in plastic wrap and refrigerate for at least 2 hours or overnight.
3. Make the filling: To a blender, add the poppy seeds and grind until they look like moist soil, about 30 seconds. Use a rubber spatula to scrape the poppy seeds into a medium saucepan. Add the honey, water, lemon zest and juice, orange zest and juice, and sea salt, and set over medium-low heat. Stir the mixture together until smooth and cook until the mixture thickens slightly and a wooden spoon scraped across the bottom of the pan leaves a trail that doesn't immediately fill in, about 10 minutes. Transfer the mixture to an airtight container and refrigerate until completely chilled.
4. Roll the dough: Lightly flour your work surface and place the chilled dough on top. Flour the top of the dough and roll it into a ⅜-inch-thick rectangle with the short end facing you. Use a pastry brush to remove any excess flour from the top of the dough, then fold the dough into thirds like a business letter, folding the top third down first and then the bottom third up and over the top. Turn the dough so the open end faces left, then roll the dough into a ⅜-inch-thick rectangle again. Repeat the three-step process 2 more times. Cut the dough in half to form 2 squares. Wrap each piece of dough in plastic wrap and chill for 1 hour or overnight.
5. Make the rugelach: Preheat the oven to 350°. In a small bowl, stir together the sugar, cinnamon and nutmeg; set aside. Lightly flour the work surface and roll a piece of the chilled dough into a 12-inch circle (place a 12-inch cake pan or the ring from a 12-inch tart pan on top of the dough and trim around it for a perfect circle). Lightly moisten the edge of the dough (about ½ inch around the perimeter) with some of the beaten egg and dollop half of the poppy seed filling over the surface of the dough, using an offset spatula to spread the filling into an even layer that just reaches the egg-washed border. Use a pizza wheel or chef's knife to divide the circle into quarters. Slice each quarter into quarters so that you have 16 small triangles. Starting at their wide ends, roll up each triangle. Place the rugelach on a parchment-paper-lined baking sheet so the small tip is on the underside of the rugelach. Repeat with the remaining triangles.
6. Brush the rugelach with the remaining beaten egg and sprinkle with the spiced sugar. Bake until the rugelach are nicely browned, about 30 minutes. While the cookies are baking, repeat with the remaining piece of dough and filling. Cool the cookies on a wire rack completely.
Make Ahead: The rugelach dough can be refrigerated for up to 3 days before baking. The filling can be stored in an airtight container for up to 1 week before using.
Storing: The cookies can be stored in an airtight container at room temperature for up to 1 week.