In a bowl of a stand mixer, combine sourdough discard, water, and flour and mix until a shaggy dough is formed. Allow to rest for 30 minutes.
Next, add the salt, sugar, butter, and yeast to the bowl and mix for about 6 minutes. The dough will be well combined, silky, and elastic. You should hear a slight "slapping" sound as the dough beats the sides of the bowl. If you do not hear it, keep mixing. Cover and let rise until doubled. This will be faster than traditional sourdough due to the instant yeast. Usually this will take about an hour or so but it depends on the temperature of your home.
Once you see that the dough has doubled in size, dump out onto the counter, smooth side facing down. Gently shape your dough by pulling it into a rectangular shape with the short side closest to you. Then, fold each side longways and roll up like a log. Tighten the top of the dough by pulling the dough towards you without lifting it.
Butter your loaf tin, then place dough in the pan. Allow to rise until the dough is slightly above the rim of the pan. The takes about an hour but depends on the temperature of your kitchen.
Now it is time to bake. Preheat your oven to 350 degrees and allow to warm for about 10 minutes. Then, place your dough in the oven and bake for about 40-45 minutes or until it is a nice golden brown.
Once it is done, remove from the oven and place on a cooling rack to cool. Once cool, turn out of pan, slice, and enjoy! This bread will keep for a few days on the counter in a plastic bag or freeze it for longer storage.
