Place the milk and butter in a small saucepan over medium flame. Stir until the butter melts and remove from the heat. Set aside to come down to lukewarm temperature.
In the bowl of a stand mixer, add 2¼ cups of the flour, the yeast, the granulated sugar and the salt. Add a dough hook and begin to knead. Add the egg and water and knead until just starting to come together. Slowly add the milk and butter mixture and then add the rest of the flour. Knead until smooth and then let it rest for 10 minutes.
In a bowl, mix all of the filling ingredients until well combined.
On a lightly floured surface, roll out the dough into a 9“x 15” rectangle. Spread the filling evenly over the entire surface of the dough. Tightly roll up the dough. Some of the filling will squeeze out but, it will get used. Cut the roll into 8-10 pieces and lay them, cut side down, into a buttered baking dish. Scoop up all of the filling that fell out and spread over the rolls. Combine the cinnamon and brown sugar and sprinkle over the rolls. Bake in a 375ºF oven for 30 to 40 minutes or until golden brown. Remove from the oven to cool.
Meanwhile, make the buttercream. In a bowl, mix all of the buttercream ingredients until well combined. When the rolls are still warm but not hot, spread the buttercream all over the rolls and serve.
Any extra buttercream should be refrigerated for another use.
You can garnish the rolls with the “un-used” Pixie Tangerine segments or just eat them. They are the best tangerines!!!