Position a rack in the center of the oven and heat it to 350°F. Lightly grease a 9 x 13-inch light-colored metal pan, then line the bottom and long sides with parchment, leaving handles for lifting. Spread the pecans on a rimmed baking sheet and toast for 6 to 8 minutes, stirring once, until fragrant and slightly darkened. Cool completely before chopping so their heat does not melt the streusel butter.

In a medium bowl, whisk the 1 cup flour, 3/4 cup brown sugar, cinnamon, and 1/4 teaspoon salt until no compressed sugar lumps remain. Stir in the cooled chopped pecans. Pour in the melted butter and fold with a fork until no dry flour remains and the mixture forms moist clumps ranging from small crumbs to hazelnut-sized pieces. Set the streusel aside at room temperature while preparing the batter.

Whisk the 2 1/2 cups flour, baking powder, baking soda, and 3/4 teaspoon salt in a medium bowl for about 30 seconds, distributing the leaveners thoroughly. In a separate bowl, whisk the sour cream, 3/4 cup maple syrup, milk, vanilla, and 1 teaspoon maple extract until smooth. Keeping these mixtures separate until needed limits unnecessary gluten development and helps the thick batter combine without pockets of flour or sour cream.

Using a stand mixer fitted with the paddle attachment, beat the softened butter and 3/4 cup brown sugar on medium speed for 3 to 4 minutes. Stop once to scrape the bowl and paddle. The mixture should look lighter, slightly increased in volume, and creamy rather than greasy. Add the eggs one at a time on medium-low speed, mixing for about 30 seconds after each and scraping the bowl so the emulsion remains smooth.

Run the mixer on low and add one-third of the dry ingredients, followed by half of the sour cream mixture. Repeat, then finish with the remaining dry ingredients. Stop the mixer while a few flour streaks remain and fold the batter with a flexible spatula, scraping the bottom and sides. The finished batter should be thick, smooth, and able to drop slowly from the spatula. Stop immediately once no dry flour remains.

Spread slightly more than half of the batter across the prepared pan using an offset spatula, pushing it carefully into the corners. Scatter one-third of the streusel evenly over the batter without pressing it down. Spoon the remaining batter across the surface in small dollops, then gently connect them so the streusel stays in place. Cover the batter with the remaining streusel, preserving both small crumbs and larger craggy clusters.

Bake on the center rack for 42 to 48 minutes, rotating the pan after about 30 minutes only if the oven browns unevenly. The cake is ready when the top is golden, the center springs back under light pressure, and a wooden pick inserted away from the streusel ribbon comes out with moist crumbs but no wet batter. An instant-read thermometer in the cake portion should register approximately 200 to 205°F.

Set the pan on a wire rack for 20 minutes, then use the parchment handles to lift the cake carefully onto the rack. Cool completely for about 70 minutes before glazing or slicing, since residual warmth would melt the glaze and compress the crumb. Whisk the confectioners’ sugar, 2 tablespoons maple syrup, 1 teaspoon milk, 1/4 teaspoon maple extract, and salt until smooth. Drizzle over the cake, let it set for 15 minutes, then cut into 12 slices.