1. Prepare the sweet potato. Slice thinly and add to a pot of water. Bring to a boil and cook sweet potatoes until tender and easily pierced with a fork. Drain, mash, and set aside.
2. Heat the milk and butter together in a small pot on low heat until the butter is completely melted. Pour the mixture into a small bowl and let cool until the mixture is not quite lukewarm, 110°F (43°C). Add the yeast and let sit for 10 minutes to proof the yeast.
3. In a large bowl, combine the mashed sweet potato and eggs, then beat until combined.
4. Add the proofed yeast to the mashed potato mixture and stir in.
5. In a separate bowl, whisk together the sugar, cinnamon, nutmeg, salt, cardamom, and 2 cups of the flour.
6. Add the flour mixture to the sweet potato mixture and stir in gently. Continue to add the remaining 2 1/2 cups of flour, 1/2 cup at a time, until the dough begins to pull away from the sides of the bowl. Note: you may not need to use all of the flour.
7. Once the dough comes together, turn it out onto a lightly floured surface and knead with your hands until the dough is smooth
8. Oil the bottom of a large bowl and place the kneaded dough in the bowl. Cover the bowl with plastic wrap or a damp towel and set in a warm place to rise.
For the filling:
1. While the dough rises, make the filling by mixing together the sugar and cinnamon together in a small bowl.
To assemble the cinnamon buns:
1. Once dough has doubled in size, turn it out on a lightly floured surface, punch it down and knead for a few minutes. Roll into a rectangle.
2. Spread the 1/4 cup of butter over the rolled-out dough, then sprinkle the cinnamon sugar filling evenly overtop.
3. Gently roll up the dough lengthwise to form a log.
4. Cut the dough into 1 inch-wide pieces and place in a greased 9x13 inch pan. Cover with a damp cloth and let rise in a warm place for 45 minutes, or until doubled in size.
5. Preheat oven to 350°F (180°C). Once risen, bake rolls for 25 minutes. Let cool and prepare the glaze.
For the glaze:
1. Combine icing sugar, milk, and salt in a small bowl, then whisk until thick and creamy.
2. Drizzle over top of the cooled cinnamon rolls. Serve immediately, or store, covered, at room temperature for up to three days, although cinnamon rolls taste best the day they're made.
