In a mixer fitted with a dough hook, combine starter, milk, flour, egg, sugar, butter, and salt.
Mix 8–10 minutes until the dough is smooth and elastic — it should pass the windowpane test.
Transfer to a lightly oiled bowl, cover, and start bulk fermentation.
During the first 60 minutes, perform 2 sets of stretch-and-folds, 30 minutes apart.
After 2 hours, gently fold in the soaked and drained raisins, pressing them evenly into the dough.
Continue bulk fermenting for another 2 hours (total bulk time = 4 hours). The dough should look puffy and airy.
Roll the dough into a rectangle (about 12x16 inches).
Mix brown sugar, cinnamon, flour, and softened butter into a crumbly paste.
Spread the filling evenly over the dough, leaving a 1-inch border at the top edge.
Roll the dough tightly from the short side into a log; pinch the seam closed and tuck the ends underneath.
Place the log seam-side down into a greased loaf pan.
Cover and refrigerate overnight (8–12 hours).
The next day, bring the dough to room temperature for 4–5 hours, until it rises about 1 inch above the rim.
Brush with egg wash.
Bake at 350°F (175°C) for 40–45 minutes, or until the top is golden and the internal temperature reads 190°F (88°C).
Cool in the pan for 10 minutes, then transfer to a wire rack and let cool at least 2 hours before slicing.