Preheat oven to 350 degrees.
Grease an 8 x 4-inch loaf pan with cooking spray.
Mix the butter and brown sugar in a stand mixer until well combined.
Mix in the banana, pumpkin, Greek yogurt, honey, eggs, and vanilla.
Sift together the whole wheat flour, baking soda, salt, baking powder, cinnamon, ginger, and nutmeg. Gradually mix the flour mixture into the batter.
Fold in the walnuts.
Pour the batter into the loaf pan. Allow the batter to rest uncovered for 15 minutes.
Bake in the oven for 55-60 minutes.
Remove from the oven and allow to cool on a wire rack for 10 minutes before serving or storing.