Make the tangzhong: in a small pot, whisk together the bread flour, water and milk. Cook on medium whisking frequently until the mix turns into a thick paste, about 5 minutes.
In the bowl of a stand mixer, add the 120g warm water and sprinkle the yeast over it. Whisk or stir gently to dissolve then let set for about 5 minutes, until it starts to foam.
Over the yeast add the flour, sugar, sour cream, salt, vanilla, oil, and the flour paste (tangzhong). Knead on medium speed until the dough starts to clean the bowl and come around the dough hook, this can take up to 15 minutes. When it's done it won't have a smooth surface but won't be overly sticky. Place the dough in an oiled bowl and cover with plastic wrap. Set it to rise on the counter for a couple of hours until it has doubled in size.
Grease a 13×9 cake pan.
Option to brown the butter: place the butter in a frying pan and cook on medium heat. It will melt, then sizzle and sputter. Once it goes quiet transfer it to a heatproof bowl making sure to scrape in the brown specks let cool until it reaches a solid but creamy state.
Mix the softened butter with the brown sugar, flour, dash of vanilla, salt, cinnamon and nutmeg. If it looks like it will be difficult to spread, warm it briefly in the microwave.
Roll out the dough onto a well floured surface to about a half inch thickness. Spread the filling onto the dough and roll it up into a log from the shorter side. Using unwaxed dental floss, slice the log into 12 rolls and arrange them in the prepared pan - if you are running out of space, bake the extra rolls in a cupcake pan. Cover the pan with a tea towel and let them rise until they are puffy and when you press them, the dough doesn’t immediately spring back (about an hour, more if your kitchen is cold).
Preheat the oven to 350 F. Bake the rolls for 30 minutes, until they are slightly browned.
When the rolls are cool, make the glaze: whisk together the powdered sugar, heavy cream (use more if the glaze is too thick), cream cheese if using, salt and vanilla. Spread over the rolls. Grate some nutmeg over the top too, if you like!
Keep the rolls in the pan wrapped in foil on the counter. To rewarm the next day, set them in the oven with the foil on and preheat the oven to 250 F, they should be warm to eat after 10-15 minutes.