Preheat the oven to 400 degrees (skip this step if doing long fermentation) and line a jumbo muffin tin with jumbo muffin liners or spray/grease well. Set aside.
In a large mixing bowl, using a wooden spoon or Danish dough hook, cream the cooled brown butter and the granulated sugar for about 2 minutes.
Then, add the eggs one at a time mixing well after each addition.
Next, add the sour cream and sourdough and mix well.
Add the vanilla extract/paste and pumpkin at this point and stir until well combined.
When the wet ingredients are well combined, measure out the dry ingredients; flour, baking powder, cinnamon and sea salt in a separate small bowl 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 will be tough.
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.
NOTE: I love doing the long ferment on these muffins. If I do it this way I usually make the streusel up at the same time as the muffin batter. This is so that I can easily get the muffins in the oven when the fermentation is done. Simply put the streusel covered in the fridge until the muffin batter is ready.