Preheat the oven to 180°C (350°F) and line the sides and bottom of the pan 20x10x6cm (8x4x3 inches) with melted butter and baking paper.
Add the softened butter and the sugar to a bowl of an electric mixer fitted with a paddle attachment. Cream the butter and sugar until light and fluffy, about 4 minutes.
Add the eggs, one at a time, to the batter, scraping down the sides of the bowl between each addition before mixing again. Once added, add the vanilla and mix again.
Sift the flour, baking powder, baking soda, cinnamon, and salt into a large bowl.
Add the mashed bananas and the flour mixture to the batter by alternating between the two. Starting with the flour and ending with the bananas. Scrape down the sides of the bowl and mix again to ensure everything is well combined and lump-free.
Add the chopped pecans to the cake batter and stir using a rubber spatula. Pour the banana bread batter into the prepared loaf tin.
Add all the streusel ingredients to a small bowl, except for the nuts. Use your fingertips to massage the cold butter, flour, and sugar together until the mixture forms a sandy consistency with larger and smaller lumps. Add the nuts and mix to combine.
Sprinkle the streusel mixture over the banana batter and bake in a preheated oven for about 50 minutes or until a skewer inserted into the center of the loaf comes out clean and the loaf is golden brown. Allow the banana bread to cool in the tin for 15 minutes before removing it from its tin and transferring it to a wire rack. Slice, serve, and enjoy.