For batter: Preheat the oven to 325°F. Butter a 9x5-inch loaf pan and line it with parchment paper, if using. Set aside. Sift the flour, baking powder, baking soda, and salt into a medium mixing bowl. Using an electric mixer, cream the butter in a separate bowl, gradually adding the sugar. Add the eggs, one at a time, beating well after each addition. Blend in the vanilla and lemon zest. In a small bowl, combine the mashed banana with the sour cream and mix to blend. Put a third of the dry mixture into the creamed butter mixture and stir to blend. Add half of the banana mixture, stir, then add another third of the dry ingredients and stir. Add the remaining banana mixture, then the remaining dry ingredients, blending thoroughly after each addition. Scrape the batter into the prepared pan and smooth the top with a spoon.
For topping: Combine the sugar, flour, butter, and pecans in a food processor and pulse until crumbly. (This can also be mixed by hand. Chop the pecans first.) Spread the topping evenly over the batter and press gently, to partially embed. Bake on the center oven rack for 60 to 70 minutes, until a tester inserted deep into the center of the bread comes out clean. Cool the loaf in the pan on a rack for 10 minutes. Turn the bread out of the pan and cool to room temperature on a rack before slicing.