Making the Tangzhong:

1. In a small sauce pan, whisk the flour and water together until there are no lumps. Cook over medium-low heat, stirring constantly, until the mixture reaches 150 degrees and then remove it from the heat. If you don’t have an instant read thermometer, remove the mixture when it is thick and the spoon begins to make trails.
2. Scrape the mixture into a clean bowl, cover, and allow it to cool completely.
3. You can make the roux ahead of time and keep it in the refrigerator for a couple of days. After the mixture cools, cover it with plastic wrap and place it in the refrigerator. If it turns gray, throw it out.

Making the Dough:

1. In the bowl of a stand mixer, mix together 250 grams (2 cups) flour, sugar, and kosher salt. Let rest.
2. In a medium sauce pan, sauté the chopped sage in the butter and olive oil until the sage is softened and the butter is golden brown. Pour in the milk and mix thoroughly. Let cool to 120 to 130 F.
3. Add the warm milk mixture to the flour. Using the dough hook, mix at low speed until there are no bits of dry flour. Blend in the tangzhong roux on low speed until thoroughly combined.
4. Add the sourdough and mix on low speed until well blended. Gradually add 125 grams (1 cup) flour, beat on low speed just until combined. Let rest 15 minutes.
5. Add the roasted sweet potato and mix on low speed until thoroughly integrated into the dough.
6. Beat at low speed until a soft, tacky dough forms. Scrape the sides of the bowl, as needed. If dough is too sticky, sprinkle in flour, 1 tbsp. at a time, as needed, up to ¼ cup.
7. Remove the dough to a large bowl. Cover, and let the dough ferment for 2-3 hours. Stretch and fold the dough, 2 times, every 40 minutes, then allow the dough to rest for an additional 1-2 hours.
8. Line a 9×13-inch baking pan with parchment paper.
9. Remove the dough to a counter lightly sprinkled with flour. Knead briefly to remove air, shape into a rough ball and let rest 5 minutes.
10. Divide dough into 12 portions, about 71 grams each. Form each portion into a ball and place seam side down in the baking pan.
11. Allow the rolls to proof for 2 hours at room temperature, then place in the refrigerator overnight for a cold ferment.
12. OR, continue to the baking phase once the rolls are fully proofed.
13. If you give the shaped rolls a cold ferment overnight, the next day, take them out of the refrigerator and allow them to warm up to room temperature.
14. Preheat the oven to 375 F., with a rack placed in the lower shelf of the oven, about 15 minutes before you plan to bake the rolls. (In my 5-rack oven it’s the 2nd shelf from the bottom.)
15. Brush the rolls with milk (or an egg wash, if preferred). Bake for 7 minutes on the lower shelf, then move rack to the middle shelf, rotate pan, and bake an additional 7 minutes. Move rack up one more shelf, then bake 1 minute more to achieve golden brown color.
16. Let the rolls cool in the pan for 5 minutes. Brush with melted butter. Serve warm or at room temperature. Store in an airtight container for up to 2 days.