Weigh your sourdough starter into your mixing bowl. Add the warm water and yeast and mix to combine.
150 grams sourdough starter, 350 grams water, 7 grams yeast
Add the flour and salt to the wet ingredients and mix until all the flour is combined. If too sticky, add a little more flour in 1 tablespoon increments.
500 grams all purpose flour, 10 grams salt
Using a spatula or your hands, scrape the side of the bowl and pull the dough into a ball. Pull the dough from each side (right, left, top, and bottom) up and toward the center. This is called 'stitching' and helps develop the gluten.
Cover the bowl with plastic wrap or a disposable shower cap (sold cheap on Amazon). Place in a warm location to double in size, about 2 hours.
Once the dough has doubled in size, punch down and shape into a log to fit your loaf pan. You'll get the most uniform loaf by stretching and folding like a burrito.
Place the loaf an oiled 9 x 5 loaf pan and cover with the shower cap again.
At this point you can refrigerate to cold ferment for 12 to 24 hours before baking, or set in a warm location to almost double in size, about 45 minutes.
Preheat your own to 400℉ and place your rack in the center of the oven.
Once your loaf has proofed to almost double, place it in the preheated oven and bake for 35 to 40 minutes. To check for doneness, an instant read thermometer should read 195℉ to 205℉ in the center of the loaf.
When finished baking, remove the loaf from the oven to a rack. Allow to cool for about 5 minutes and then turn out, placing on the rack again. Using a tablespoon, smear 2 tablespoons of butter over the hot crust top of the loaf.
2 tablespoons butter
Cool completely before slicing.