Make the dough: Whisk together the eggs, yolk, granulated sugar, oil and milk in the bottom of a stand mixer bowl. Add the flour, yeast and salt and mix with the dough hook on low until it comes together, then let the machine knead it for about 5 minutes. Scrape the dough onto a lightly floured counter just long enough to coat a mixing bowl with oil, then return the dough to the oiled bowl to rise until doubled, about 2 hours.
For the filling and assembly: Butter a 13-by-9-inch (or equivalent size) baking dish, or coat it with nonstick spray.
Combine the cream cheese, granulated sugar and vanilla in a medium bowl (I just use a fork to mix this).
On a very well-floured counter, roll out the dough into a rectangle about 16 inches wide (on the side facing you) and as far away from you as it comfortably goes, usually 13 to 15 inches. Spread the cream cheese mixture all over the rolled dough. Dollop the jam over it and spread it smooth. Roll the dough into a snug spiral; it will stretch to about 18 inches wide.
Cut the log very gently--it's going to be a soft mess, so use a sharp serrated knife or dental floss--into 1 1/2-inch segments. Arrange each in the prepared baking dish. Brush the tops of the buns with the egg wash and cover with plastic wrap. Refrigerate overnight, along with the leftover egg wash (also covered).
The next morning, remove the buns from the fridge and let warm up for 30 minutes. Preheat the oven to 350 degrees F.
Brush the tops of the buns with the egg wash again and sprinkle with poppy seeds. Bake until the buns are bronzed all over and have an internal temperature of 190 degrees F, about 30 minutes. Let cool slightly before serving.
For the glaze (if using): Whisk the powdered sugar together with 3 tablespoons lemon juice until smooth, drizzling in an additional tablespoon of lemon juice as needed to loosen. Drizzle this over the buns, or serve it alongside with a spoon.