For the dough: Warm the milk and 1/2 cup water in a medium saucepan over low heat to about 100 degrees F (but no more than 110 degrees F). Remove the saucepan from the heat, and sprinkle the yeast over the liquid. Sprinkle a pinch of the sugar over the top, and set aside without stirring until foamy, about 5 minutes. Then whisk in the butter and egg yolk.
Whisk together the flour, remaining sugar and salt in a large bowl. Make a well in the center, and stir in the yeast mixture with a wooden spoon until you have a thick, slightly sticky dough. Turn the dough onto a floured work surface, and knead until soft and elastic, about 6 minutes. Shape the dough into a ball.
Brush the inside of a large bowl with butter. Add the dough ball, turning to coat lightly. Cover the bowl with plastic wrap, trace a circle the size of the dough on the plastic and note the time. Let the dough rise in a warm place until doubled in size, about 1 hour 30 minutes.
Turn the dough out of the bowl, and knead briefly to release excess air. Re-form it into a ball, and return it to the bowl. Lightly butter a large piece of plastic wrap, and lay it on the dough. Cover the entire bowl tightly with the rest of the plastic, and refrigerate for 4 hours or overnight to let the dough proof.
For the filling: Preheat the oven to 350 degrees F. Spread the poppy seeds, sesame seeds, garlic flakes and onion flakes on a pie plate, and toast until lightly browned and fragrant, about 3 minutes. Let cool for 5 minutes. Mix the cream cheese, egg and flour in a small bowl until combined. Stir in the scallions, lemon zest, 1 teaspoon salt and a few grinds of pepper.
To fill and form the buns: Butter a 9-by-13-inch baking pan with the 2 tablespoons softened butter. Turn the dough onto a floured work surface, and press it flat. Roll the dough into a 10-by-18-inch rectangle, with a long side facing you. Spread the filling evenly over the dough, leaving a 1-inch border on the side opposite you. Set aside 2 tablespoons of the toasted seed mix (for the topping), and scatter the rest over the filling. Starting from the side facing you, roll the dough up into a tight roll. Lightly brush the 1-inch border with water, and continue rolling to seal the roll. Fold the ends in just a bit, and if any spots look thicker than others, even out the roll with your hands.
Cut the roll every 1 1/2 inches with a serrated knife, to make 12 buns. Place the buns cut-side down (except for the end pieces) in the prepared pan, spacing them 1 inch apart. Cover the buns loosely with plastic wrap, and set in a warm place to rise until they double in size with no gaps in between, 1 hour 30 minutes to 2 hours.
For the topping: Stir the butter, reserved seed mix and scallions in a small bowl; set aside.
Position an oven rack in the center of the oven, and preheat to 350 degrees F. Bake the buns until they're golden brown and the tops spring back when pressed lightly, 25 to 30 minutes. Spread the topping evenly over the buns, sprinkle with 1 teaspoon salt and return to the oven until the butter is melted and the topping is fragrant, 5 minutes more. Let the buns cool in the pan for 10 minutes before serving.
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