In the bowl of a stand mixer fitted with a dough hook attachment (or use a hand mixer with hook attachments and a large mixing bowl), combine the flour, yeast, sugar, and salt. Mix to combine.

Add the egg, milk, and sweet potato purée. Knead on medium-low for 3 minutes. Then switch to medium and knead for 3-5 minutes, or until the dough forms and pulls away from the sides of the bowl.

Add the butter one tablespoon at a time and knead until fully incorporated.

When all the butter is incorporated, knead on medium for 5-7 minutes, or until you can stretch a small piece of the dough into a square, creating a thin film that allows light to pass through (windowpane test). The dough should look hydrated, silky, and smooth, but still sticky.

Cover the bowl with plastic wrap and leave it in a warm place to proof for one hour (an oven with the light on works well).

Line a 9" x 14" (23cm x 35cm) baking pan with parchment paper or grease it with butter.

When the dough has risen significantly and is jiggly, transfer it to a floured work surface.

Divide the dough into 15 even pieces (approximately 60g each, or eyeball it). Shape each piece into a tight ball and place them in the prepared baking pan. If the dough is too sticky, dust it and your hands with some flour before forming each ball.

Arrange the dough balls in the pan and cover them with plastic wrap. Leave at room temperature to proof for 40-60 minutes, or until doubled in volume and jiggly.

Preheat the oven to 190C (375F).

Bake the buns on the middle rack for 15 minutes, or until the internal temperature reaches 190F (88C). The tops should be a deep golden brown, and the bread should sound hollow when tapped on the bottom of the pan.

Remove from the oven and let the rolls cool in the pan.