In the bowl of a stand mixer or a large bowl, combine melted butter, flour, sugar, and a cup sourdough starter.
Mix well with a silicon spatula until all the flour is coated and a ball forms. Since this is a quick bread, you do not need to knead the bread.
Cover bread dough with a towel or plastic wrap. Let the dough rest at room temperature for 8-24 hours.
The next day, preheat the oven to 350 degrees. Grease the bread pan well or line with parchment paper.
Heat a skillet over low heat and add butter. Allow the butter to melt.
Add diced apples.
Cook the apples down, stirring frequently until they start becoming translucent and soft.
Add brown sugar and cinnamon. Stir until coated with butter and it starts to caramelize and the sugar is completely melted. Be careful to not burn the apples. This is fairly easy to do.
Turn off heat and allow to cool for a few minutes.
To the bowl with the fermented dough, add eggs, vanilla extract, salt, baking soda, baking powder and cinnamon.
Mix until well incorporated.
Slowly add in the slightly cooled caramel apple mixture into the batter. I like to do this with the stand mixer on low gently pouring the apples in a little bit at a time. This helps it incorporate better without running the risk of cooking the eggs.
Mix together until combined, scraping down the sides occasionally.
Pour into the prepared loaf pan.
Bake for 55-65 minutes until it starts to brown and a toothpick comes out clean. If it is getting too brown but has not baked all the way through yet, cover it with parchment paper or aluminum foil.
Allow to cool before cutting into it.
Serve with a little butter, or for an extra decadent dessert, some ice cream.