First, brown the butter see above for how to make brown butter.
Preheat the oven to 400 degrees** (skip if doing the long ferment method) and line a muffin tin with muffin liners or spray well with cooking spray. Set aside.
In a large mixing bowl, using a wooden spoon or Danish dough hook, cream the brown butter, that has been cooled, and the granulated sugar for about 2 minutes.
Then add the eggs, one at a time, mixing them well after each addition.
Next, add the sour cream and mix well.
Add the vanilla extract and sourdough starter at this point and stir until well combined.
When the wet ingredients are well combined, measure out the dry ingredients; flour, sea salt and baking powder and add those on top of the wet.
Just barely mix the dry ingredients into the wet and then add the whole milk. Finish mixing until it is just combined. Do NOT overmix or your muffins might turn out dense.
At this point you will either let the muffins ferment for 5 hours on the counter or 2-4 hours on the counter and the rest of the time overnight in the fridge. You can also just skip the long ferment and bake the muffins immediately.
The next morning, preheat the oven to 400 degrees (350 degrees for mini muffins) and finish the rest of the recipe.
Gently fold in the chocolate chips.
Next, scoop the dough evenly between the 12 regular muffin liners, 6 jumbo muffin liners or 24 mini muffins depending on which ones you plan to make.