For the bread:

1. Butter two 9x5 inch loaf pans or use vegetable spray.
2. Mix yeast, a pinch of sugar, and ¼ cup warm water in the bowl of the stand mixer, and let stand until foamy, about 3 minutes.
3. Add the sweet potato purée, buttermilk, oil, sugar, egg, zest, and 1 cup of flour to the yeast mixture and mix with the paddle attachment until it's all integrated.
4. Change to the dough hook attachment and mix at medium speed, adding half a cup flour at a time, until the dough starts to combine and come together. The dough will be slightly sticky and not completely clear the sides of the bowl. Be careful not to add too much flour or you will end up with a dry dough. 
5. Transfer to an oiled bowl, cover with plastic wrap and let rise in a draft-free place until doubled, about 1 ½ hours. Check at half-hour intervals to see how much it's rising. If the room is warm it might take less. 
6. Transfer to a lightly floured board. The dough will be less sticky. 
7. Deflate gently and roll (lightly flour the rolling pin) make a rectangle about ¼ inch thick. You might need to let it rest a few minutes between rolls to get it to the right thickness as it might be slightly elastic and shrink back. But you will be able to roll it a bit further with each resting period. 
8. Brush evenly with melted butter and sprinkle with the spiced sugar. It might seem like a lot but the result is wonderful. Carefully start rolling from the long side, as you would a cinnamon roll, trying to get a fairly tight rope.
9. Cut the dough in half and place each piece into one pan.
10. Sprinkle with the streusel (recipe below), dividing evenly. 
11. Cover with a clean kitchen towel and let rise again until puffed, about 45 minutes.
12. Preheat oven to 350ºF / 180ºC about 20 minutes before baking.
13. Bake loaves until golden and a tester inserted comes out clean, from 45 minutes to 1 hour. 
14. Let stand a few minutes after you remove them from the oven, and then invert them onto a wire rack. If you don’t do this while the bread is warm, the caramelized rim will harden and make it impossible to remove it later. You’d have to pop them again into the oven for 5 minutes to loosen it up if this happens.
15. Keep leftovers well wrapped in the freezer, already cut in slices preferably if you plan on toasting them.

For the streusel:

1. Mix all ingredients except butter in the food processor. Or in a bowl if making by hand, with a fork or pastry cutter. 
2. Add butter and mix until it starts to clump. Reserve, covered, in the refrigerator. 

For the spiced sugar:

1. Mix all ingredients in a bowl.