Pumpkin and butter are two different ingredients. Do not substitute the one for the other in cake unless you do not care about the change of texture and flavor. You will still have cake, but it will not be as tender or, well, buttery.
3/4 cup dark brown sugar
3/4 cup pumpkin pie filling (This was a subsitute for butter.)
3/4 cup molasses
3/4 cup boiling water
2 cups all purpose flour
2/3 c masa arepa corn flour (This was a subsitute for all regular flour.)
1 1/2 teaspoons ginger
1 1/2 teaspoons cinnamon
1 teaspoon salt
1 teaspoon baking powder
1 teaspoon baking soda
(I left out 1 teaspoon grated orange peel and 1/2 teaspoon allspice.)
I baked 6 mini-bundt cakes in a (preheated) 325*F oven for 25 minutes.