Add active starter, brown sugar and warm milk to the bowl of a stand mixer with the paddle attachment. Be sure that the milk isn’t boiling (no hotter than 110 F) – this could kill the starter and prevent the dough from rising.
100 grams active sourdough starter, 30 grams light brown sugar, 175 grams milk, warm
Mix on low speed until the ingredients are mixed thoroughly.
To the bowl, add melted unsalted butter, orange zest, salt, and an egg. Mix on low until fully combined.
57 grams unsalted butter, melted, 5 grams orange zest (optional), 5 grams salt, 1 egg
Switch the paddle attachment with the dough hook attachment. While mixing on medium, add the bread flour, about 70g (1/2 c) at a time, until incorporated. Stop to scrape the sides of the bowl with a rubber spatula in between each addition of flour.
370 grams bread flour
Once all of the flour has been added, continue to knead the dough for 5 more minutes at medium speed.
For the first rise, cover the bowl with plastic wrap or a reusable shower cap and let the dough rest in a warm spot to double in size. I love using this bowl to help me know when my bulk fermentation is complete! In my 70 degree F kitchen this took about 6-7 hours. The rise times can vary depending on the temperature of your kitchen. The warmer your kitchen, the faster the dough will rise. The opposite is true for cooler kitchens. Note: If you prefer to shape the rolls the following day, after the dough has completed its bulk ferment, place the covered dough in the fridge overnight. Continue with the recipe the following morning.