The night before, remove the sourdough starter from the fridge and feed it. Allow it to sit n the counter all night.
In the morning, the starter should be active and bubbly. If it isn't, feed it again and it should be ready in a couple of hours.
Measure the active starter that you'll need, feed your main starter, and return it to the fridge until you want to bake with it again.
In the bowl of a stand mixer (or a large mixing bowl to knead by hand), cream the softened butter on medium speed.
Turn the mixer off and add in the active sourdough starter, honey, eggs, buttermilk, and salt. Mix on medium speed until ingredients are well combined.
Turn the mixer to low and add in the flour, 1 cup at a time, until the dough is smooth and only slightly tacky.
Cover the dough in the bowl with a damp tea towel and allow it to sit in a warm place until double in size, about 4 or 5 hours.
Remove the ball of dough and using a bench scraper or spatula, cut it in half. Cut each half into 8 equal pieces.
Generously grease two loaf pans with butter. Take each piece of dough, roll it into a ball, and place in the loaf pan, 4 on each side for 8 total in each loaf pan.
Cover with a damp towel and allow to rise in a warm place until doubled in size, about 2 or 3 hours.
Preheat the oven to 350 degrees F.
In a small bowl, whisk 1 egg with a splash of water until foamy. use a pastry brush to generously brush the egg wash all over the tops of the loaves.
Bake for about 25 to 30 minutes or until golden brown on top. If the tops begin to brown too much, just lay a piece of aluminum foil on top of them and allow the loaves to continue baking.
Cool on a wire rack completely.