Mix the starter ingredients in a clean jar. Leave to peak in a warm spot, around 6 hours, until at least doubled.
Combine the flour, sugar, and salt in a large bowl. Add the water, milk, and active starter and mix into a shaggy dough.
Turn out onto a lightly floured surface and knead for a few minutes to bring it together.
Add the softened butter a few small cubes at a time, kneading them in. Continue kneading for 6 to 8 minutes until smooth.
Place the dough in a lightly greased large bowl, cover, and leave to bulk ferment in a warm spot (around 25°C / 77°F) for 3 to 4 hours, until risen by around 30-40%.
For an overnight ferment, cover tightly and refrigerate for 8 to 24 hours.
Line a 9 x 13 inch baking dish with baking paper. Turn the dough out and divide into 12 equal pieces (around 75 to 80g each).
Roll each piece into a 40cm rope on a lightly floured surface, dusting with more flour if it sticks. Shape into a U, cross the ends over, and tuck each end through the loop to form a knot. Place in the baking dish.
Cover and proof in a warm spot until almost doubled and noticeably puffy with a soft jiggle when nudged, around 3 to 4 hours.