Prick 1 medium sweet potato all over and bake at 400F for 50 to 60 minutes, until it is collapsing and a knife meets no resistance. Never boil it, because boiled flesh carries water you cannot measure into the dough.

Split it, scoop out the flesh, mash it completely smooth and measure ¾ cup. Let it cool until it is warm rather than hot.

Whisk the warm milk, instant yeast and brown sugar in the bowl of a stand mixer and let it stand 5 minutes until foamy.

Add the sweet potato mash, the whole egg and the extra yolk, then 3 cups of the bread flour and the salt. Mix on low with the dough hook until it comes together, then knead 6 minutes on medium low.

With the mixer running, add the soft butter one tablespoon at a time, letting each piece disappear before adding the next. The dough will look broken for a few minutes. Keep going.

Knead another 6 to 8 minutes until smooth and glossy. The dough should pull cleanly from the sides of the bowl while still clinging to the bottom. If it is still smearing up the walls, add the reserved flour 1 tablespoon at a time, waiting a full minute between additions.

Scrape the dough into a lightly oiled bowl, cover, and let it rise 60 to 90 minutes until doubled. A floured finger pressed half an inch in should leave a dent that fills back slowly and only part way.

Divide into 12 equal pieces, about 85 g each. Cup your hand over each piece and drag it in small circles on an unfloured counter until the surface is smooth and taut, then flatten each ball to about 3 inches across.

Space the buns on two lined sheet pans and prove 40 to 45 minutes, until puffy and just touching at the edges.

Brush all over with the egg wash beaten with the milk, right down the sides, and add sesame seeds if using. Bake at 375F for 18 to 20 minutes, until deeply golden and 190F inside.

Cool on a rack at least 30 minutes before slicing, so the crumb finishes setting.