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

Add the salt and softened butter to the dough and knead for about 10 minutes until smooth and elastic.

Cover the bowl with a damp cloth and let it rest for 1-2 hours, or until doubled in size.

Once the dough has risen, turn it out onto a lightly floured surface 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 a cloth and let them rise for another hour.

Preheat the oven to 375°F (190°C).

Bake the buns for 20 minutes, or until golden brown on top.

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