In the bowl of a stand mixer, whisk the yeast, warmed whole milk, and 1 teaspoon of sugar. Let it sit for 5-10 minutes, until the mixture is frothy.

Next, add the remaining sugar, vanilla extract, the egg, and the unsalted butter in chunks. You will see streaks of egg and clumps of butter, but that's ok! Add the salt and 3 cups of flour, mixing over medium-low speed until the dough starts to come together. Add more flour 1/4 cup at a time until the dough forms into a soft, shaggy ball and is just slightly tacky. Don't add too much flour!

Knead the dough over medium-low speed until the dough is smooth and holds its shape, about 4-6 minutes. Smooth the dough into a ball and place it in a greased bowl. Cover the bowl with a linen, and place it in a warm spot to rise until doubled, about 1-2 hours.

Grease a 9×5-inch loaf pan and set aside. Then, stir together the cinnamon and brown sugar until smooth. Set aside.

Punch down the risen dough to release the air. On a lightly floured surface, roll out the dough into a 9×18-inch rectangle. Spread the very soft butter over the dough, leaving about a 1-inch gap on all sides. Sprinkle the cinnamon-sugar mixture evenly over the butter.

Starting with the narrow (9-inch) side of the dough facing you, tightly roll up the dough into a loaf. Pinch the ends to seal, and place the loaf seam-side down into your greased pan.

Cover the loaf pan with a linen, and let it rise in a warm spot until the dough is puffy and creates a dome-shape over the pan, about one hour.

Preheat the oven to 350°F. In a small bowl, whisk the egg with 1 Tablespoon of water and set aside.

Then, in another small bowl, whisk together the flour, sugar, cinnamon, and salt. Add the melted butter and mix until clumps form. Set aside.

Brush the loaf with the egg wash and sprinkle with the streusel. Form the streusel into large chunks, or break it up with your fingers for finer crumbs. (You may have some leftover).

Bake the bread for 35-40 minutes, until the loaf is golden-brown and makes a hollow sound when you give it a tap. Let the bread cool for about 20 minutes in the pan. Then, gently remove the loaf from the pan and let it cool completely on a wire rack.

Slice the bread and serve it with plenty of butter! Store the leftovers covered in the loaf pan for up to 3 days. Or, wrap it well, place it in an airtight container, and freeze the bread for up to 2 months.