In the bowl of your stand mixer, combine sourdough discard, water and 510 g flour. Before setting the mixing bowl on the stand mixer, hold the dough hook and stir to combine. Then, attach the bowl and dough hook, and mix these ingredients on speed 2-3 until it turns into a dry ball of dough (about 30-60 seconds.)
Cover with a kitchen towel and let dough rest for 30 minutes, in the meantime measure out the remaining ingredients.
After 30 minutes, add in yeast, salt, sugar, and butter. Continue kneading on stand mixer for 5-10 minutes. Add in flour a tablespoon or two at a time until dough comes together and butter no longer is seen and is fully incorporated.
Kneading is complete when you have a smooth ball of dough that generally swirls along staying attached to the dough hook. Turn off mixer, remove dough and shape into a smooth ball. Gently poke/push the dough in with a finger and watch for it to spring back. If it pops back quickly it is a good sign the dough is done kneading.
Place dough in a greased, clear container, or any container that will be easy to recognize it has doubled in size. I prefer to use this one, with the numbers. Rise until at minimum doubled in size (approximately 1.5-2 hours, depending on how warm your house is.) Instant yeast will make it rise quickly so keep an eye on it!
After it has doubled in size, remove to the counter and flatten and stretch the dough with your hands until it makes a large rectangle. Mentally divide into thirds, and fold the far right 1/3 over the center 1/3. Lastly, fold the left hand 1/3 section onto the 2/3 section, so it is another long skinny rectangle with 3 layers stacked neatly on top of one another.
Tightly roll the dough from the bottom of the just formed skinny rectangle, tucking the dough under itself basically as you go. You'll need to stretch and flatten the dough as you roll to keep it similar to the shape of your loaf pan. I love the USA pullman loaf pan with cover, but you can use whatever loaf pan you prefer! Once it is your desired shape and size, tuck the rough ends under to smooth it out and place seam side down into a greased loaf pan.
Allow to rise for another 45-60 minutes approximately until once again doubled in size. Watch closely in this final rise, as you will want to preheat your oven when it looks like it is nearly risen.
Place in preheated 350°F oven, and bake for 50-60 minutes, until internal temperature reaches 190°- 210℉. Remove to wire rack immediately to allow to cool. Try not to cut the warm bread, I know it is tempting but it is best to let it cool for multiple reasons. You're done, you did it!!!