Mix the dough: In the bowl of a stand mixer add the flour, water, salt, sugar, milk, sourdough starter, water, and optional active dry yeast. Mix on low speed with the dough hook attachment for 1 minute. Add the softened butter and chocolate chips and continue mixing, scraping down the sides as needed, until a smooth dough forms (about 3-4 minutes).
605 g all-purpose flour, 66 g granulated sugar, 12 g kosher salt, 60 g sourdough starter, 214 g water, 120 g whole milk, 60 g mini chocolate chips, 1.5 g Optional: active dry yeast, 57 g unsalted butter