1. Before you begin: This recipe is written for a 13x4 loaf pan. If you are using a different size, use the calculator above this recipe card for a revised ingredient list. Additionally, a ton of ingredient substitutions have been tested. Please see the substitutions section for tips. If you are mixing by hand (no stand mixer) I have a recipe post dedicated to those instructions. See the Notes section for the link!

The Night Before

1. Make the stiff sweet starter: In a small jar, mix 30 g sourdough starter, 30 g sugar, 60 g water, and 120 g bread flour until it forms a ball. Knead outside of the jar to fully incorporate the ingredients. Cover loosely and let ferment at room temperature until morning.
   

Bake Day

1. Make the tangzhong: Mix 35 g bread flour and 175 g whole milk in a microwave-safe bowl. Microwave in 20-30-second intervals, stirring between each, until thickened and reaches 150°F (65°C), or about 4-5 rounds total. Let rest on the counter for about 30 minutes, or until the mixture has cooled below 100°F (38°C). Note: you could make this the night before and refrigerate overnight.
   
2. Mix the dough: In your stand mixer bowl, combine All of the stiff sweet starter, All of the tangzhong, 195 g cool water, 40 g sugar, 45 g avocado oil, 515 g high-protein bread flour, and 12 g salt. Mix on speed 1 (lowest speed) for 1-2 minutes until ingredients come together, then increase to speed 2 (a tiny bit faster) and mix for 10-15 minutes, or until dough reaches a weak windowpane. It takes about 7 minutes in the Ooni, 10 minutes in a Bosch, and upwards of 30 minutes in a KitchenAid, for reference.

   Hand mixing instructions are in the Recipe Notes.
   
3. The dough will feel slightly sticky or tacky after mixing, but it should hold its shape. If the dough doesn't fully come together after a few minutes in the stand mixer, add additional flour in 1 tbsp increments. Resist adding too much, though, or the resulting loaf will be dense.
4. Bulk fermentation: Let dough rise in a warm place (~85°F/29°C) until risen by about 30% and small bubbles appear around the sides of the bowl (about 3-5 hours).
   
5. Shape the dough: Spritz your loaf pan with a nonstick oil spray. Dump dough onto work surface (use a light dusting of flour to prevent sticking, if needed). Using a rolling pin, roll dough into rectangle slightly narrower than pan width (about 10" wide by 14" long). Try to pop large bubbles as you roll. Roll up the dough tightly, turning the ends in slightly as you roll to minimize an end seam. Place your dough log seam-side down in loaf pan. Use a toothpick to pop any bubbles on the surface of the dough.
   
6. Second rise: Cover with plastic wrap and let rise in your warm spot (~85°F/29°C) until very puffy and the dough is even with the height of the pan rim (approximately 3-5 hours).
   
7. Bake: Preheat oven to 350°F (175°C). Bake uncovered 40-45 minutes until internal temperature reaches 200-205°F (93-96°C). (See Notes for different loaf pan size baking times.) If the top of the loaf is getting too dark, cover with foil.
8. Finish: Once done, immediately brush hot loaf with 1 tbsp butter. Cool in pan 5-10 minutes, then turn out onto rack. Cool completely before slicing (1-2 hours).