Heat oven to 350°F. Generously grease a Bundt pan (at least a 9-cup pan).
Melt 1 cup of butterscotch chips over low heat on the stove or in the microwave, stirring until smooth. Be careful not to get the chips too hot or they will seize up and harden. Remove from heat and set aside.
In a large bowl, mix together the flour, sugar, baking powder, cinnamon, pumpkin pie spice and salt.
In a smaller bowl, whisk together the pumpkin, oil, eggs, vanilla and melted (and cooled to room temperature) butterscotch chips.
Gradually stir the pumpkin mixture into the flour mixture, stirring until well combined.
Streusel
In a small bowl, combine the brown sugar, chopped pecans, cinnamon, and pumpkin pie spice.
Pour half of the cake batter into the Bundt pan. Toss the streusel mixture evenly over the cake batter. Pour the rest of the cake batter over the streusel.
Bake at 350° for 50-60 minutes for a metal pan (or 60-65* minutes for a stoneware pan), or until toothpick or cake tester comes out clean. Remove from oven, turn the Bundt pan upside down onto a cooling rack and let cool for 15 minutes before lifting the pan off the cake. When cake is completely cool, sprinkle with powdered sugar.