Begin by making the dough. In a stand mixer fitted with a dough hook combine the yeast and warmed milk. Let sit for 10 minutes until foamy. Add in the sugar, eggs, 4 ½ cups of flour and salt. Starting the mixer on low, gently knead the dough together. Once it comes together add in the softened butter in chunks. Add more flour if the dough looks too sticky. Knead the mixture on one step above low for about 10 minutes or until it begins to make a slapping sound in the bowl and looks smooth.
Remove from bowl and gently knead it on a lightly floured surface, about 10 times. Roll it into a ball and place it in a well oiled bowl. Cover with plastic wrap and a clean kitchen towel. Place in a warm area for about 1 ½ to 2 hours or until it doubles in size.
Grease a 9x13 baking sheet or cookie sheet. Set aside. In a small bowl combine the brown sugar, cinnamon and cornstarch.
Remove the risen dough from the bowl and divide in half. Gently roll out the dough on a floured surface with a rolling pin. Roll it into a rectangle roughly the same size as the sheet pan. Spread half of the softened butter over the dough and sprinkle with half of the filling mixture. Roll the dough on top of itself starting at the long end and rolling up. Cut the roll into 8 even sized rolls. Place the cut rolls on the greased sheet pan, cut side up. Repeat with the other half of the dough.
Cover the sheet tray with plastic wrap and place in a warm area until puffy and slightly risen, about 30 minutes. Or, cover with plastic wrap and refrigerate overnight. The next morning let the rolls come to room temperature, about 45 minutes, before baking.
Preheat oven to 375° F and place an oven rack in the middle.
Bake the rolls for 20-25 minutes or until golden brown and cooked through. A toothpick inserted into the middle of the rolls should come out clean.
Remove from oven and let cool slightly. While the rolls are cooling make the frosting. In a stand mixer or with a hand mixer combine the butter, vanilla, powdered sugar and milk. Beat until smooth and fluffy.
Once the rolls have cooled, liberally spread the frosting over the tops. Will keep covered, at room temperature for 3-4 days.
Recipe Adapted from Williams Sonoma