Step 1Heat the oven to 325 degrees. Melt one-half cup butter in a heavy skillet over medium heat. After it melts, continue to cook, whisking until it turns nut brown, about 8 minutes. Remove the browned butter from the heat and cool, then refrigerate until it solidifies, about 30 to 40 minutes.
Step 2While the browned butter is chilling, put the pecans on a jellyroll pan in a single layer and toast them in the oven 15 to 20 minutes. Remove the nuts from the oven and cool in the pan, then roughly chop them so the pieces are no larger than one-fourth inch.
Step 3For the pecan streusel filling, combine one-half cup chopped toasted pecans, 1 1/2 teaspoons flour, and one-third cup brown sugar. Work the remaining 2 tablespoons butter into the sugar mixture until it is crumbly; do not over mix.
Step 4Cream together the chilled browned butter and the remaining 1 cup brown sugar until light and fluffy, 2 to 3 minutes. Beat in the eggs, one at a time. Scrape down the sides of the bowl, then beat in the vanilla.
Step 5Sift together the flour, baking powder, baking soda and salt. Add the dry ingredients to the brown butter mixture alternately with the sour cream, folding each addition in gently by hand. Stir in the remaining 1 1/2 cups chopped toasted pecans, just until ingredients are mixed.
Step 6Spoon half the batter into the bottom of a well-buttered 9-inch loaf pan. Sprinkle the streusel filling evenly over the batter. Spoon the remaining batter over the filling and spread evenly. Bake 55 to 60 minutes or until the bread tests done in the center; note that the streusel filling will remain moist throughout the baking process. Remove to a wire rack and let cool to warm. Serve warm or at room temperature.