Mix active sourdough starter, brown sugar, water, and flour in a small bowl. Knead into a bowl and let sit for several hours, until doubled in size.
Once starter dough has doubled in size, add to a stand mixer with paddle attachment. Add cooled, scalded milk, egg yolks, softened butter, sugar, and vanilla. Mix until just combined.
Once combined, switch to a dough hook. Add one cup of flour at a time and only right after the cup before is just combined. With the last cup of flour, add 1/2 tsp salt. Mix 4-5 minutes until dough is smooth.
Your dough will be soft and pliable but not sticky. Add to another bowl, cover, and let rest 8-12 hours (preferably overnight), until dough is doubled in size once again.