Put milk in stainless pan. Cut vanilla bean in half and scrape seeds into milk; drop scraped beans into milk. Heat over medium until milk is barely scalding, then turn off heat and cover. Let steep at room temperature for at least one hour. Can be made ahead of time and refrigerated. Makes enough for four loaves.
Measure ¾ cup of vanilla-steeped milk in microwave safe bowl. Add butter, then warm slowly in microwave just until butter melts. Make sure it is just warm to the touch; if it gets too hot, leave it at room temperature until it cools before proceeding.
Mix, flour, brown sugar, yeast, and salt in bowl of bread machine. Whisk together eggs and yolks, then pour into bread machine bowl on one side. Onto the other side, pour milk mixture. Let bread machine run on dough setting until it finishes the first rise. Remove the dough from the machine, shape into a smooth ball by rolling edges underneath and pinching, and drop into panettone mold. Let rise in warm, steamy place until the dough rises 2 to 3 inches above the edge of the pan.
Before proceeding, preheat oven to 325° convection and adjust rack to lower third of the oven. Prepare egg wash and assemble tools and ingredients because the next step needs to happen quickly.
With hands, gently spread a thin layer of egg wash over the top of the risen dough. Sprinkle with sanding sugar. With extremely sharp knife or razor, cut a large, deep X into the top and place a small pat of butter into the middle of the X. Place on baking sheet and put into preheated oven.
Bake 30 to 45 minutes, until golden brown and top sounds hollow when tapped. Internal temperature should be around 185°.
Remove from oven and cool upside down. This can be done by inserting a wooden skewer horizontally through the paper mold, and suspending the bread between two cardboard boxes or hanging using twine. Leave hanging until bread is completely cooled.
Notes: this is a very finicky recipe due to the very high fat content: even if you do everything exactly right, the bread may overpuff and/or and collapse anyway. Plan to make extra loaves in series just in case. The collapsed bread is still delicious even if it’s not beautiful. This recipe was finessed for baking in Davis, California, which is near sea level and often has very low humidity.
