Preheat the oven to 350 degrees (F). Grease the Bundt pan well with butter and sprinkle sugar in it as you would flour to prevent sticking.
Melt the 1/2 cup of butter and set aside to cool.
In a bowl, mix together the flour, baking powder, baking soda, salt, cinnamon, and nutmeg. Set aside.
In a large mixing bowl, combine 1/2 cup melted butter and the oil, and mix. Beat in the eggs. Add the sugar and dark brown sugar and beat on slow, then gradually increase the speed. Add the pumpkin and vanilla and beat.
Slowly add in the flour mixture, about 1/3 cup at a time.
Pour mixture into the greased Bundt pan.
Bake 55 minutes or until a cake tester comes out clean. Allow to rest on a rack about 15 minutes, then loosen the edges and middle and flip onto a serving plate.
Optional: sprinkle with powdered sugar before serving. Serve warm or cold.
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