In a large bowl, whisk the cream, ¼ cup plus 2 tablespoons lukewarm water, the sugar, and the yeast. Add 2 of the eggs and whisk until smooth. Add half the flour and 1¼ teaspoons kosher salt and whisk until the mixture is smooth, shiny, and stringy, about 2 minutes. Swap the whisk for a wooden spoon and add the rest of the flour, stirring until fully incorporated into a very soft dough. Scrape the dough from the sides of the bowl, then cover the bowl with a heavy towel. Set aside at room temperature until the dough rises a few inches (it won’t double), 60–80 minutes. (When poked, the dough will be soft but shouldn’t stick to your finger.)
Meanwhile, prepare the filling by combining the bacon and onion. Season with salt and pepper.
Preheat the oven to 425°F. Grease 2 baking sheets with a thin layer of lard or butter and set aside.
In a small bowl, beat the remaining egg with 1 teaspoon cold water and set aside. Generously dust a work surface and rolling pin with flour. Turn the dough out onto the surface, sprinkle a little more flour over it, then roll out to a 1⁄3-inch-thick rectangle, about 15x12 inches. Brush the top of the dough with the egg wash, then use a long knife to cut it lengthwise into 5 even strips. Divide each strip into 4 pieces, making twenty 3-inch squares. Top each square with 2 tablespoons of filling. Wrap one corner of each dough piece over its filling, and pinch the edges shut. Roll each on a lightly floured surface to make a fat dumpling with pointed, tapered ends. Bend the bun into a crescent shape and transfer to the baking sheets, leaving about 2 inches between each.
Brush the tops of the buns with more egg wash and bake for 10 minutes, then lower the heat to 350°F and continue baking until the tops are a dark, golden brown, 20–25 minutes. Serve warm.