Heat the oven to 400F. Pierce a sweet potato several times with a knife. Place on a baking sheet or wrap in foil. Bake at 400F for 45 minutes to an hour, until completely soft. Turn off the oven. Remove the skin and any stringy bits. Mash with a potato masher or fork until very smooth, then measure out ½ cup of mashed sweet potato.
Stir together warm water, yeast, and sugar in a large mixing bowl. Cover with a dish towel and let sit for 10 minutes, or until yeast is foamy and activated.
Add ½ cup mashed sweet potato, canola oil, and salt to the yeast mixture. Stir until smooth. Add in flour one cup at a time, stirring with a large spoon or stand mixer. Continue mixing until 3 cups of flour are added. The dough should be shaggy and slightly sticky, but easy to work with.
Turn dough out on to a lightly floured surface and knead for 5-6 minutes, until elastic and slightly tacky. If the dough is too sticky to work with, continue adding flour ¼ cup at a time, up to 4 cups total.
Place the dough in a lightly oiled bowl, turning to coat. Cover with a dish towel and let rest for 1 hour, or until doubled in size.
Return dough to a lightly floured surface, punching down to deflate. Divide into 6 equal pieces using a bench scraper or knife. Use a scale for exact pieces, or just eyeball it (that's what I usually do). Gently flatten each dough piece, then pinch all 4 corners into the center. Fip and gently roll each ball with the palm of your hand to form a smooth bun shape.
Place each bun on a parchment paper-lined baking sheet. Preheat the oven to 350F. Cover the buns and let rise for 15-20 minutes, or until the dough has noticeably increased in size. Bake sweet potato buns on the middle oven rack for 25-30 minutes, or until they are lightly browned. Remove from oven and set on a rack to cool.
Store buns in an airtight container at room temperature for 2-3 days, or freeze for up to 3 months.
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Notes
Water should be no more than 110F or the yeast may die. I've found that 106-108F is an ideal temperature for happy yeast. If you don't have a thermometer, use your hand to test the water. It should be just hot enough that you can leave your hand under the water without it burning, like a hot bath tub.
Substitute half of the all purpose flour for whole wheat flour if you prefer. 100% whole wheat is a bit harder to work with, so I usually keep it half white/half wheat.