Preheat the oven to 350°F. Grease the loaf pan with shortening and then, if desired, the bottom with parchment paper. Set aside.
In a large mixing bowl or the bowl of a stand mixer fitted with a paddle attachment, cream together the shortening and sugar until light and fluffy. Add the vanilla, eggs and mashed bananas and blend on low until incorporated.
In a separate medium bowl sift together the flour, baking powder, baking soda, cinnamon and nutmeg. Gradually add the dry ingredients to the wet mixture until combined. Do not overmix.
By hand, fold in the walnuts. Lightly coat a 9-inch bread pan with cooking spray and line bottom with parchment paper for easy removal. Transfer batter into bread pan.
Bake for 30 minutes then remove to add the streusel topping. Return to the oven for an additional 25-30 minutes, checking for doneness using the toothpick test. If it starts to brown on the top, loosely over with a leaf of aluminum foil. Remove and allow to cool for at least 30 minutes before removing from the loaf pan and cutting into slices using a serrated knife.
In a small bowl, combine the flour, brown sugar, salt and cinnamon.
Using the back of a fork, blend in the softened butter until it makes large clumps. Sprinkle this topping over the top of the banana bread at the 30-minute marl.
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