Mix the Dough: Weigh out your ingredients using a kitchen scale and add to the bowl of a stand mixer: active sourdough starter, water, oil, honey, salt, and egg. These can be mixed together by hand with a whisk or the paddle attachment on your mixer.
Add in Flour: Once mixed, switch to your dough hook and with the mixer on low speed begin adding in flour, one cup at a time. The dough should look rough and shaggy, if it's still wet and sticky add more flour 1/2 cup at a time. Then drop in the softened butter in tablespoon size pieces
Knead the Dough: Begin kneading the butter into the dough on medium to low speed with the dough hook attachment. Continue to knead for 5-7 minutes. The dough should feel soft and not stick to your hands. You can test the dough with the windowpane test
Bulk rise: Transfer the dough to a large bowl and cover with plastic wrap or a lid for an overnight rise. Or allow to rise in a warm spot until doubled in size.
Shaping the dough: Once doubled in size, remove the dough from the proofing bowl, and transfer to a lightly floured surface. Divide the dough into three equal pieces. Gently shape each piece into a log the size of your loaf pans, tucking the ends underneath.
Second Rise: Transfer each loaf into a parchment lined bread pan smooth side up. Cover with plastic wrap or a damp towel for a second rise until slightly domed over the top of the loaf pan.
Baking the loaves: Once the dough has risen, preheat your oven to 400℉. Bake the loaves for 28-32 minutes. The tops should be golden brown and the internal temperature should reach 180℉.
Cool the Loaves: Remove from the oven and place on a cooling rack. Be sure to remove the parchment paper while cooling, this will trap steam and make the bottom of your loaf soggy.
Storing the Sandwich Bread: Once each loaf is cool, store it in a plastic bag on the counter for 5-7 days or freeze for up to 6 months.