In a small bowl, combine the yeast with the lukewarm water and 1/2 teaspoon of granulated sugar. Set aside for 5 minutes or until the yeast bubbles up.
In the bowl of a stand mixer, add butter, sugar, and 1 cup of hot water and beat for 30 seconds, using the beater blade. Let cool to lukewarm temperature. Stir in 2 cups of flour and mix until smooth. Add yeast mixture and mix until well combined. Mix in the beaten egg.
Gradually stir in the remaining flour and mix with the dough hook for about 2 minutes. Remove dough from the bowl and place on a lightly floured counter. Knead by hand, add a little flour if the dough is still sticky. Knead until dough feels satiny and smooth.
Cover and let rise for 30 to 60 minutes or until the dough has doubled in size. The time will vary depending on how warm your kitchen is.
While the dough is rising, make the streusel. In a large bowl, stir together the flour, brown sugar, salt, cinnamon, and melted butter. You can combine with your hands after combined to form clumps. Put in the refrigerator until ready to use.
After the dough has doubled in size, remove it from the bowl. With a rolling pin, roll the dough into a rectangular shape, about 16 X 12 inches.
Spread dough evenly with the filling: softened butter, brown sugar, and cinnamon. Top the filling with half of the streusel.
Roll up dough into one long roll. Cut rolls, using a piece of dental floss or thread, about two inches thick. Place rolls in a greased 9X13-inch baking pan, leaving about 2-inches between the rolls so they have room to rise. You should have 12 cinnamon rolls.
Cover the pan with a clean kitchen towel. Let the rolls rise until double in size. This should take 45 to 60 minutes.
When the rolls have doubled in size, sprinkle the remaining streusel over the cinnamon rolls. It should completely cover the cinnamon rolls.
Preheat oven to 350 degrees F. Bake for 30 minutes or until cinnamon rolls are golden brown on top and cooked in the middle. Remove from the oven and let the rolls cool to room temperature.
While the rolls are cooling, make the frosting. In a medium bowl, whisk together butter, confectioner’s sugar, milk, and vanilla. Drizzle the frosting over the cinnamon rolls. You can also put the frosting in a piping bag or Ziploc bag (cut a small hole in the bottom) and pipe the frosting onto the rolls.
Serve immediately. Enjoy!