In a large bowl, mix together the cake and pudding mix, pumpkin, oil, eggs, water and pumpkin pie spice.
Stir in the butterscotch chips and pour batter into a bundt pan sprayed heavily with non-stick cooking spray.
Bake for 45-50 minutes, or until top is springy to the touch and a wooden toothpick inserted comes out clean. Cool cake thoroughly in pan for about 90 minutes before inverting onto a plate. If desired, dust the cake with powdered sugar or top with the best brown sugar frosting.
For the frosting:
In a small saucepan, melt butter over medium low heat.
Mix in the brown sugar and cook and stir for 2 minutes.
Stir in the milk and bring to a boil, stirring the entire time.
Remove from the heat and stir in the sugar.
Let cool for a couple of minutes and then spread on top of a cooled cake.