The day before baking, prepare the sourdough: combine 70 g sourdough starter with 300 g wholemeal wheat flour and 300 g lukewarm water. Cover loosely and leave at warm room temperature for 12–16 hours until bubbly and active.
The next day, in a large bowl, combine 600 g of the active sourdough with 300 g wholemeal rye flour, 375 g spelled flour (type 630), 1 tbsp barley malt flour, 20 g cane sugar, 20 g salt, and 5 g soft butter. Gradually add up to 220 ml lukewarm water while mixing — different flours absorb differently, so stop adding water once the dough is smooth and slightly tacky.
Knead for 8–10 min until elastic. Cover the bowl with a clean cloth and let rise in a warm, draft-free spot for at least 1 hour, until noticeably risen.
Turn the dough onto a lightly floured surface and divide into 16 equal portions. Shape each into a round roll by tucking the edges under and rolling under a cupped hand.
Line two baking sheets with parchment. Arrange the rolls well-spaced on the sheets, cover with a cloth, and let rise in a warm place for another hour, until puffy.
Preheat a fan oven to 175 °C (345 °F). Place an ovenproof dish of water on the oven floor for steam.
Bake both trays for 15–20 min, until the rolls are deep golden and sound hollow when tapped on the bottom. Swap the trays halfway through if your oven bakes unevenly.
Transfer to a wire rack and let cool completely before serving.