Preheat the oven to 425 degrees F. Spray a baking sheet with nonstick spray, or line with a silpat mat.
In a large bowl, whisk together the flour, brown sugar, baking powder, salt and baking soda. Cut in the 1/2 cup butter until the mixture resembles coarse crumbs. In a small bowl, stir together the pumpkin and buttermilk; add to the crumb mixture and stir just until moistened.
Turn the dough onto a lightly floured surface; knead 8 to 10 times, adding a small sprinkle of flour here and there if needed. Pat or roll out to 1-inch thickness; cut with a floured 2-inch (or similar) biscuit cutter. Place cut biscuits 1-inch apart on a greased baking sheet (or a baking sheet lined with a silpat mat).
Bake for 18 to 22 minutes or until golden brown. Melt the remaining 1 tablespoon butter in a small bowl. Brush the melted butter over the biscuits. Serve warm.