This recipe was created using a stand mixer (Thermomix). See notes for info on kneading by hand.Add warm milk, water, sugar and instant yeast to the bowl of your stand mixer. Mix together until the sugar has dissolved and the yeast is foamy. Now add your sourdough starter, bread flour and salt. Bring the ingredients together to form a shaggy dough. This just means that the liquid and flour are brought together - the dough will look a bit dry at this stage. Now, using the dough hook of your stand mixer, knead the dough for a few minutes until it becomes smoother.
Add the butter and continue to knead the dough until it is smooth and elastic. This dough is not a sticky dough and should some together in one big ball.This should not take too long using a stand mixer - no more than 5 minutes once the butter is added. Remove the dough from your stand mixer, place into a bowl and cover with plastic wrap. Leave the dough to rise until it's doubled. The dough must double - if it doesn't, your buns won't be as fluffy. Once the dough has doubled, turn it out onto the counter and form into a rough rectangle. Using a dough scraper, cut the dough into 9 strips (less if you want larger rolls). Using a scale, weigh out each piece, adding or removing dough until you get 9 pieces that are 100g each.
Shape each piece of dough into a log and place on a baking sheet. Leave a little gap between them - but you want them to be touching once they've risen.
Preheat your oven to 180C/356F and set the rolls aside for a second rise - you want them to be puffy (it won't take too long - around 30 minutes to an hour).
Once the rolls are puffy and touching each other, brush them with some milk.
Place into the oven and bake at 180C/356F for around 15 minutes. The rolls should be lightly browned on top.
Take the rolls out of the oven and brush generously with butter. You don't have to melt the butter, just brush it on with a pastry brush and the warmth from the rolls will melt it.
Place the rolls onto a wire rack to cool.