Combine the melted butter and milk in a small bowl. Cool slightly before using.
Add the egg, sourdough starter, and sugar. Mix to combine.
Add the flour and salt. Continue mixing until a rough, sticky dough forms, about 1 minute.
Scrape down the sides of the bowl. Cover with a damp towel and let rest for 30 minutes.
Let rest 30 min and do a set of stretch and folds (20 plus).
Let rest 30 min and do another set of stretch and folds (4 to 10) until the dough feels soft, supple and pulls away from the bowl - add extra flour if it is sticky.
Let rise until double in size, about 8-12 + hrs.
Place risen dough on floured surface and stretch it into a rectangle (approx 12 x 16).
Spread filling evenly on dough leaving a ½ around the edges clear.
Starting on the long side of the dough (16-inch), roll it into a log pressing down gently as you go.
Cut the dough into 8 same size pieces.
Place the pieces swirl side up in a pan lined with parchment paper.
Let it rest for 1 to 2 hrs so the dough puffs up.
Bake at 350 for 30 to 40 min.
Let it cool for 15 min to let the butter soak in.
Dust with powdered sugar or glaze with a cream cheese icing.
For the glaze, beat softened butter, whipped cream cheese and sifted powdered sugar until smooth, thinning out the consistency with a little milk as needed.