In the bowl of a stand mixer, combine the milk, starter, egg, butter, granulated sugar, salt, and flour. Mix for 7 minutes to combine. Alternatively, knead by hand until smooth. Dough should be soft, smooth, and slightly tacky.
1 cup milk, 1/2 cup sourdough starter (125 g), 1 large egg, 4 tbsp butter, 1/4 cup granulated sugar, 1 tsp salt, 3.5 cups flour (525 g)