To make the dough: Warm the milk in a pot or microwave (to a maximum temperature of 110 degrees Fahrenheit/43 degrees Celsius). Place in a mixing bowl and sprinkle yeast on top. If using active yeast, let sit for a few minutes and watch to ensure that it begins to foam. If it doesn’t foam, throw it out and start again. (If you’re using instant yeast, you can skip this step.)

Once the yeast foams, add sugar, egg, yolk, and butter. Mix until well-combined. Next stir in the flour and salt with a wooden spoon until a dough begins to form.

Knead the dough for eight to 10 minutes. It should form a nice ball and still be slightly sticky. If the dough is too sticky to knead, add flour one tablespoon at a time. You don’t want to add too much flour. When the dough is ready, it should bounce back when poked.

Transfer the dough to a well-greased bowl. Cover with plastic wrap and a dishtowel and allow to rise for one and 1/2 hours, until approximately doubled in size.

While the dough is rising, combine sugar and cinnamon for the filling in a small bowl.

Transfer the dough to a piece of Gefen Parchment Paper and top with a second sheet of parchment. Roll out dough between the sheets into a 12×18-inch (30×46-centimeter) rectangle. Smear dough generously with softened butter and cover completely with the cinnamon-sugar mixture.

Roll the dough tightly, making sure to seal the edges as best as possible. For small buns, roll up from the longer side. For larger buns, roll from the shorter side. Using a serrated knife, cut the roll into one-inch (two and 1/2 centimeter) slices.

Line a square or round baking pan with parchment paper. Place the buns into the pan, making sure to leave room between them to allow for rising. Cover the pan with plastic wrap and a towel again and allow to rise for another 30 minutes.

Preheat oven to 350 degrees Fahrenheit (175 degrees Celsius). Remove the towel and plastic wrap and place the baking pan in the oven. Bake the rolls for 20–25 minutes. You want them to be soft and slightly underbaked in the center, since they continue to bake as they cool.

While the rolls are baking, prepare the frosting: With an electric mixer, beat all ingredients until smooth and fluffy. Allow buns to cool for about 10 minutes and then spread frosting over top. These are best eaten very fresh!