Pour milk into microwave safe bowl and warm for about 40 to 45 seconds. The milk should feel like bath water, about 110° F. Transfer warm milk to bowl of stand mixer and sprinkle yeast on top.
Add in sugar, egg, egg yolk, and melted butter. Mix well, until combined.
Stir in flour with wooden spoon until dough begins to form.
Place dough hook on mixer and knead for about 8 minutes. Dough should form a nice ball and be slightly sticky. If dough is too sticky to handle add 2 tbsp of flour.
Transfer dough to a well-oiled bowl, cover with plastic wrap and a warm towel and allow dough to rise for 1 to 1 1/2 hours in dry, warm area. This could take more of less time depending on the temperature and humidity of your home.
Once dough has doubled in size transfer to a well-floured surface and roll into a 14x9 inch rectangle.
Then spread softened butter over dough leaving about a 1/4 inch margin around dough edges.
In a separate small bowl, mix together brown sugar and cinnamon. Use your hands to sprinkle mixture over buttered dough. Then pat brown sugar mixture into butter so it sticks.
Starting from the 9-inch side tightly roll up dough. Place the seam side down making sure to seal the edges of the dough as best as possible. Cut off about an inch of dough at each end and discard.
Using serrated knife or floss cut dough into one-inch pieces, you should get about 9 cinnamon rolls.
Place cinnamon rolls in a greased 9x9 inch baking pan or 9-inch cake pan lined with parchment paper.
Cover with plastic wrap and a warm towel, let rise again for 30-45 minutes.
For Overnight Cinnamon Rolls: Cover with plastic wrap and place in the refrigerator. In the morning remove cinnamon rolls from the refrigerator and let sit on the counter for 30 to 45 minutes.
Preheat oven to 350° F. Remove plastic wrap and towel. Bake cinnamon rolls for 20 to 25 minutes or until just slightly golden brown on the edges. Allow them to cool for 5-10 minutes before frosting.