Preheat oven to 350F. Coat a 9-inch pan with nonstick spray.
Stir together the brown sugar, cream and salt for the caramel. Pour into a prepared pan and spread to coat the bottom. Sprinkle pecans over the caramel.
In a food processor, process brown sugar, flour, butter, cinnamon and the salt for the streusel. Pulse until small clumps form; set aside.
Whisk buttermilk, sour cream, pumpkin puree and eggs together and place into a large measuring cup with a pour spout. Sift flour, baking powder, baking soda, and salt into a small bowl. Cream the butter and sugar with a mixer just until combined.
Alternately add dry and wet ingredients, starting and ending with the dry. Blend each additions just until incorporated. Spread half the batter over the caramel, then sprinkle with half the streusel. Carefully spread remaining batter over that and top with remaining streusel.
Bake 50-60 minutes, or until a toothpick comes out clean. Cool cake for 5 minutes, then run a pairing knife around the sides to loosen. Invert onto a serving platter while hot, then let cool slightly before slicing.