Preheat your oven to 425 degrees F and place a cast iron skillet in the oven to heat up.
While your oven and skillet heat up, prepare your biscuit dough. In a large bowl, whisk together the flour, baking soda, baking powder, salt and sugar. Set aside.
In the pitcher of your blender or food processor, add 6 tablespoons of the sliced butter. Add the flour mixture on top. Cover with the lid and pulse 8-10 times until coarse crumbles have formed (If you don't have a blender or food processor, just use your hands and crumble until coarse crumbles form - This will take a couple of minutes). Transfer back to the bowl.
In a small bowl, whisk the pumpkin puree and buttermilk together.
Create a well in the middle of your flour mixture, and add the pumpkin mixture to it. Fold the ingredients together until a soft dough forms. Do not over-mix.
Remove the skillet from the oven (carefully), and add the remaining 2 tablespoons of the butter to the pan. Swirl the butter until melted. Spoon 1/4-1/3 cup of the biscuit dough straight onto the skillet, until you have used up the dough (I had 10 biscuits total - this will vary). Bake for 15-17 minutes or until the biscuits just begin to brown on top.
While the biscuits are baking, prepare your brown butter. In a skillet or saute pan (one without a dark bottom preferred), add the 1/4 cup of butter, and heat over medium-low to medium heat. Melt and continue to cook and stir until the butter just begins to brown. This will take 4-5 minutes. Quickly turn the heat off once you see it start to brown and a nutty aroma fills the air. Be careful, as the butter as capable of burning very quickly. Set aside.
Remove from oven and let rest for a couple of minutes before serving. Drizzle the brown butter on top and sprinkle with rosemary. Best served warm out of the oven, but will keep for 2-3 days in an airtight container.