Preheat oven to 350 degrees F (175 degrees C). Grease a 5x8-inch baking dish.
Place sweet potato into a large pot and cover with water; bring to a boil. Reduce heat to medium-low and simmer until tender, about 20 minutes. Drain and mash sweet potato until smooth.
Sift flour, white sugar, and baking powder together in a large bowl. Whisk eggs and vanilla extract together in another large bowl.
Melt butter in a saucepan over medium heat; cook and stir pumpkin seeds and brown sugar in hot butter until brown sugar is melted, 1 to 2 minutes. Stir bananas and mashed sweet potato into pumpkin seed mixture. Remove from heat and cool slightly.
Stir banana mixture into egg mixture until well-combined. Pour banana-egg mixture into flour mixture and stir until batter is just combined; stir in raisins. Pour into prepared baking dish.
Bake in the preheated oven until a toothpick inserted into the center of the cake comes out clean, about 1 hour.