YEAST DOUGH 

1. Preheat an oven to 160 degrees. Turn the oven off and crack the door. Liberally grease a large mixing bowl and set aside.
2. Dissolve  the yeast in the warm water in a large mixing bowl of a stand mixer, or use a hand mixer. Let it stand for 5 minutes. Add 1 cup of the flour, sour cream, egg, sugar, salt, and unsalted butter to yeast mixture. Mix on medium speed until everything is combined and smooth, about 45 seconds. Gradually add more flour, 1/2 cup at a time, mixing well after each addition. Continue to add the flour and mix until a soft dough forms. It should be soft and no longer wet, slightly sticky at most. I used a total of 3 1/2 cups of flour and used the remaining 1/2 cup of flour when kneading the dough.
3. Turn the dough out onto a lightly floured work surface and knead it with your hands until it’s smooth and elastic and you can easily form it into a ball, about 3 to 4 minutes. Add extra flour as needed. Form the dough into a smooth round ball.
4. Place the dough ball in the greased bowl, flipping it over so that the top is greased too. Cover the bowl with a lint free towel and place it in the warm oven. If the oven has cooled to lukewarm, you can go ahead and close the door, otherwise, leave it cracked. Allow the dough to rise until almost doubled in size, about 1 hour and 15 minutes. You can also do a poke test to see if it’s ready. Poke your finger in the dough, just like if you were poking someone on the arm. If the dough springs right back, it’s not ready. It needs a few more minutes to rise. If the dough slowly springs back like it’s waking up, then it’s ready. It will leave just a slight indentation. If the dough doesn’t spring back at all, you’ve proofed it too long.
5. Punch down the dough and transfer it to a lightly floured work surface. Roll the dough out into a long rectangle. I rolled it to a size of about 18-inches long by 9-inches wide. Yours doesn’t have to be the exact size as mine. Just try rolling it out pretty thin. You don’t want it really thick. It’s going to proof again later and puff up. 
6. FILLING: Use a pastry brush or your fingers and brush or smear the butter all over one long side of the dough, (see picture above). Sprinkle the brown sugar over the butter, followed by the cinnamon. Gently fold the remaining long side over onto the brown sugar/cinnamon side, and slightly press it down (see picture above).  
7. Using a sharp knife or pizza cutter, cut strips across the dough, about 1-inch wide (see picture above). Twist strips twice and place 2-inches apart on greased or lined cookie sheets,  pressing the ends down. 
8. Cover with a towel and place the cinnamon twists back in the lukewarm oven or any warm place in your kitchen that’s free of drafts. Allow them to rise until almost doubled in bulk, about 1 hour.
9. Preheat oven to 375 degrees. Bake the cinnamon twists until they are golden brown on top, 12 to 15 minutes. Transfer to a wire rack to slightly cool.

VANILLA GLAZE

1. In a large measuring cup, whisk together all the ingredients. If the consistency is a little too thick, add a tiny bit more milk, no more than a teaspoon at a time. If it’s too thin, add a little more sugar. It should be thin enough to drizzle from a spoon.
2. Drizzle the glaze over the slightly warm cinnamon twists and serve.
3. Cover leftovers and eat within 2 days. Homemade yeast breads are best eaten the same day. I suggest placing them in a microwave the next day for about 8 seconds, to slightly warm and soften without melting the glaze.