n Have all the ingredients at room temperature.  Position a rack in the center of the oven and preheat to 350º.   nn Butter the bottom and sides of a 10-inch springform pan.  Sprinkle the bottom of the pan with breadcrumbs. nn In a large bowl, whisk together the flour, sugar, and salt until well blended, cut in the butter until the mixture resembles coarse crumbs.  Remove 1 cup of the crumbs to a separate bowl.  To this mixture cut in 2 additional tablespoons of butter, and set aside.nn Add to the main mixture in the large bowl the baking powder and baking soda and mix thoroughly.  nn Add the buttermilk, egg, and vanilla, and beat vigorously until the batter is smooth and fluffy, 1 1/2-2 minutes.  nn Scrape the batter into the prepared pan and smooth the top with a spatula.nn Add to the reserved crumb topping the pecans, brown sugar, and cinnamon.  Toss with a fork until blended.  nn Sprinkle the crumbs over the batter and bake until a toothpick inserted in the center comes out clean, 50-65 minutes.  Let cool in the pan on a rack for 5-10 minutes.  Remove from the pan and let cool on a rack for 1 ½ hours before serving.nn  n