Wash a large sweet potato, make perforations with a fork and cover with foil. Bake it in the oven at 350°F for an hour or until soft. Remove and let cool.
Stir together flour and salt in a large bowl and make a well in the center. Add the yeast, maple syrup and a couple tablespoons warm water to activate it. Leave for 5 minutes.
Pour the kefir, sourdough starter in the well with the yeast and mix it in with just a bit of flour to begin “sponging”.
When the sweet potato has cooled to room temperature, scrape the contents out of the skin and into the sponge and mix in.
If you have a mixer with a dough hook, attach it and begin kneading on the lowest speed. If it forms into a ball you’re about ready but if it looks like batter add flour, 1/4 cup at a time until it forms a ball.
It should be a little sticky but you should be able to handle it easily. If it is too wet add more flour a tablespoon at a time, if it is too dry add more water a tablespoon at a time. This is easiest in a mixer with a dough hook.
When you have a ball, form the dough into two oblong boulés and pinch at the bottom.
On a couple large cookie sheets with a silpat or parchment paper, divide your boulés in half, and then again and again until you have rolls about 1/2 the size you desire. Fold them inside themselves into balls and pinch at the bottom to make smooth rolls.
Dust with flour and cover with tea towels and place in a warm (but off) oven, about 100° F. Let rise 45 minutes to 1 hour or place in the fridge overnight to further culture and rise.
Remove and preheat oven to 375° F
Bake in the oven at 375°F for 18-20 minutes