1. Make the dough (night before): In a large bowl or stand mixer, whisk together the starter, warm milk, eggs, and sugar. Add the flour and salt, and mix until a shaggy dough forms. Knead for 5–7 minutes, then gradually incorporate the softened butter, one tablespoon at a time, until the dough is silky and elastic. This may take 10–12 minutes of mixing. Cover and let rise at room temp for 4–6 hours, then transfer to the fridge overnight for a slow ferment (this also makes shaping easier).

2. Make the filling: In a saucepan, combine poppy seeds, milk, honey, and butter. Simmer for 5–8 minutes, stirring frequently, until thickened into a spreadable paste. Stir in cinnamon, vanilla, salt, and citrus zest if using. Let cool completely before using.

3. Roll and fill: Roll the chilled dough into a large rectangle (about 10×16 inches) on a lightly floured surface. Spread the cooled poppy seed filling evenly over the dough, leaving a ½-inch border. Roll it up tightly from the long edge into a log.

4. Shape the babka: Using a sharp knife, slice the log in half lengthwise to expose the swirled filling. Twist the two halves together, cut sides up, to form a braided loaf. Transfer to a parchment-lined loaf pan and cover loosely. Let rise at room temp until puffy and risen, about 3–4 hours (or refrigerate again for a slow second rise and bake in the morning).

5. Bake it up: Preheat your oven to 350°F (175°C). Brush the loaf with egg wash (1 egg mixed with 1 tbsp water) and bake for 40–45 minutes, until deeply golden and a toothpick inserted into the center comes out clean or with just a few moist crumbs. If desired, brush with warm simple syrup right out of the oven for that glossy bakery finish.

6. Cool, slice, devour: Let the babka cool in the pan for 10 minutes, then transfer to a wire rack. Try to wait at least 30 minutes before slicing (good luck with that).