In a stand mixer fitted with a dough hook or large mixing bowl, add the whole milk, sweet levain, egg, sugar, vanilla, and all of the tangzhong. Whisk ingredients together quickly, it's okay if it's lumpy.
All of the tangzhong, ¾ (200g) cup sweet levain , ¾ (180g) cup whole milk, ¼ (50g) cup granulated white sugar, 1 large egg, 1 tsp vanilla extract
Sourdough Discard Option: If you're making this recipe with discard and yeast, add the discard and instant yeast instead of the sweet levain to the wet ingredients and whisk in.
3/4 (200g) cup sourdough discard, 2¼ (7g) tsp instant yeast
Add your flour and salt into your wet ingredients.
4 (500g) cups all-purpose flour , 1½ (10g) tsp fine sea salt
Mix on low until a shaggy dough forms, about 2-3 minutes. Scrape down bowl if needed. If using your hands, knead until you work dough together. This is a higher hydration dough, so be aware it will be sticky.
Once the dough has come together add the softened butter, a tablespoon at a time mixing each tablespoon in until adding the next. Use a dough scraper to scrape the edges of the bowl to make sure everything is getting mixed in.
¼ (57g) cup unsalted butter
Scrape edges with a dough spatula to make sure everything is getting mixed
Continue mixing on medium-low (speed 2–3) for 8–10 minutes, until dough is smooth, elastic, and tacky but not sticky. If mixing by hand knead dough until butter has fully incorporated and dough is smooth and tacky.
Mixing Notes: If kneading in a mixer or if dough is very wet when mixing by hand, add an additional 25 grams (3 Tablespoons) of flour and mix into dough. Make sure to work flour in completely and assess if you need more flour.
Do not add more than 50 grams of flour at this stage, or you will lose the high hydration of the dough and end up with a dense donut. If using a stand mixer, dough can become more warm and sticky and I have found kneading dough with my hand (perform a set of stretch and folds) for a minute after mixing in my stand mixer really helps bring the dough together.