Preheat your oven to 400 degrees
Melt the butter and add it to a stand mixer or bowl.
Next add the brown sugar to the melted butter and mix for about 2 minutes.
Add the dry ingredients to a large bowl (AP flour, baking soda, baking powder, salt) and mix well and set aside.
First, mix the pumpkin puree, vanilla, eggs, and then, add sourdough starter and continue mixing on a low speed until it's fully incorporated.
Next, slowly mix in the dry ingredients until the dough comes together.
Stir in the chocolate chips and chopped walnuts and any other add-ins you'd like.
Take your muffin tin and add muffin tin paper lining to each cup, or use parchment paper.
Fill each cup about halfway, and cook in the oven at 400 degrees for 10 minutes.
Stick a fork or a toothpick in the center of one of the muffins, and if it comes out clean then they are done.
Once the muffins are done baking, leave them out to cool for about 5 minutes.