Preheat an oven to 350 degrees F (175 degrees C). Grease 2 loaf pans, 8 1/2 x 4 1/2 inches.
To make the streusel topping, mix 1/4 cup of cut-up butter, 1/2 cup flour, brown sugar, 1 teaspoon cinnamon, and oats in a bowl until crumbly. Set aside.
Cream the sugar and butter together in a bowl, and stir in the eggs. Add the bananas, buttermilk, and vanilla extract, and stir until the mixture is well blended. Beat in the flour, pumpkin pie spice, baking powder, cinnamon, and nuts.
Scoop the batter into the prepared loaf pans. Sprinkle the top of each loaf with half the streusel topping.
Bake in the preheated oven for 1 hour and 15 minutes, until the loaves have risen and the topping is browned. Let the banana bread cool in the pans for 5 minutes before turning out onto a wire rack to finish cooling.
