To toast pecans, preheat the oven to 325°F. Spread the nuts out on a rimmed baking sheet and bake, stirring often, until fragrant and lightly toasted, about 10 minutes.
Position a rack in the middle of the oven and preheat to 350°F. Butter and flour a 9" springform pan or a 9" cake pan with 3" sides.
To make the streusel, in a bowl, combine the flour, brown sugar, cinnamon and salt. Toss in the butter and, using 2 table knives or a pastry cutter, cut it into the dry ingredients until the mixture looks like coarse crumbs. (Or you can whir the mixture in a food processor) Stir in the pecans.
To make the batter, in a bowl, sift together the flour, baking powder, baking soda, cinnamon, ginger, nutmeg and salt. In the bowl of a mixer fitted with the paddle attachment, beat the butter and brown sugar on medium-high speed until well combined. Beat in the eggs, one at a time, scraping down the sides of the bowl with a rubber spatula. Add the pumpkin purée and sour cream and mix with the spatula. Stir in the flour mixture. The batter will be quite thick. Spread half of the batter in the prepared pan. Sprinkle half of the streusel over the batter. Dollop the remaining batter over the streusel and spread the thick batter as best you can. Top with the remaining streusel. Bake until a toothpick inserted into the center of the cake comes out clean, about 50 minutes.
Let cool in the pan on a wire rack for about 15 minutes. Remove the sides from the pan and slide the cake onto the rack.
To make the glaze, in a small bowl, whisk together the confectioners' sugar, milk and vanilla. Drizzle over the top of the cake. Cut into thick wedges.