Preheat oven to 350 degrees F. Line twelve 2 1/2-inch muffin cups with paper bake cups; set aside. For topping, in a small bowl combine the 1/3 cup brown sugar and sour cream; stir in pecans. Set topping aside.
In a large bowl stir together flour, baking powder, pumpkin pie spice, baking soda, and salt. Make a well in the center of flour mixture. In a medium bowl combine egg, buttermilk, pumpkin, the 2/3 cup brown sugar, and melted butter. Add pumpkin mixture all at once to flour mixture; stir just until moistened (batter should be lumpy). Spoon batter into the prepared muffin cups, filling each nearly full. Spoon topping evenly over batter.
Bake for 20 to 25 minutes or until golden brown and a toothpick inserted in the centers comes out clean. Cool in muffin cups on a wire rack for 5 minutes. Remove from muffin cups; cool completely on wire rack.
Transfer muffins to a storage container; cover tightly. Tote muffins at room temperature.