A marvelous hybrid of coffee cake and banana bread, that's this moist loaf with its nutty cinnamon-brown-sugar-pecan-streusel topping.
Preheat oven to 350 degrees with a rack in lower third. Butter a 9-by-5-inch loaf pan; line with parchment, leaving a 1-inch overhang on long sides. In a bowl, combine flour, brown sugar, cinnamon, and salt. Stir in butter and pecans until small clumps form and mixture is evenly moistened. (Streusel can be made ahead and refrigerated, covered, up to 3 days.)
In a large bowl, whisk together flour, baking soda, and salt. In another bowl, whisk together butter, granulated sugar, eggs, vanilla, and buttermilk; stir in bananas. Make a well in flour mixture and pour banana mixture in. Stir together until just combined (do not overmix). Spoon half of batter into prepared pan. Sprinkle half of streusel evenly over batter. Add remaining batter, then sprinkle remaining streusel evenly over top.
Bake until golden brown and a tester inserted in center comes out clean, about 1 hour, 20 minutes. (If top is browning too quickly, tent with foil.) Let cool in pan 20 minutes, then transfer using parchment to a wire rack. Let cool completely before serving.