In a mixing bowl, combine the bread flour, sourdough starter, water, salt, sugar, and softened butter. Mix until a shaggy dough forms.

Knead the dough on a lightly floured surface for about 10 minutes until smooth and elastic.

Place the dough in a lightly greased bowl, cover with plastic wrap, and let it rise at room temperature for about 1-2 hours, or until doubled in size.

Once the dough has risen, punch it down and divide it into 4 equal pieces.

Shape each piece into a round bun and place them on a baking sheet lined with parchment paper.

Cover the buns with plastic wrap and let them rise for another 30-45 minutes until puffy.

Preheat the oven to 375°F (190°C).

Bake the buns for 15-20 minutes, or until golden brown and hollow sounding when tapped.

Remove from the oven and let cool on a wire rack before slicing.