Preheat oven to 350°F (175°C). Line a 9x9-inch metal baking pan with parchment paper, leaving generous overhangs on opposite sides for easy removal. Lightly spray parchment with nonstick spray.

Make the Streusel Topping: In a medium bowl, whisk together 1 cup flour, 2/3 cup brown sugar, 1.5 tbsp cinnamon, and salt. Pour in melted butter and toss with a fork until large, cohesive, buttery clumps form. Set aside.

Make the Cinnamon Center Swirl: In a small bowl, whisk 1/3 cup brown sugar, 1 tbsp cinnamon, and 1 tsp cocoa powder until combined. Set aside.

Make the Cake Batter: In a medium bowl, whisk together flour, baking powder, baking soda, and salt.

In a large mixing bowl, beat softened butter and granulated sugar with a mixer on medium-high speed for 2 to 3 minutes until pale and fluffy. Add eggs one at a time, followed by vanilla extract, beating until smooth and fully incorporated.

Add half the dry ingredients to the butter mixture, followed by the sour cream, then remaining dry ingredients. Mix on low speed just until combined and no flour streaks remain (do not overmix). Batter will be thick.

Assembly: Spread half of the thick cake batter into the prepared pan using an offset spatula. Sprinkle the cinnamon swirl mixture evenly over the batter. Dollop remaining batter over the top, gently smoothing to cover. Evenly scatter the chunky streusel topping over the entire surface, gently pressing crumbs into the batter so they adhere.

Bake at 350°F (175°C) for 42 to 48 minutes, until a toothpick or wooden skewer inserted into the center comes out clean or with a few moist crumbs.

Cool in the pan on a wire rack for 25 minutes. Whisk powdered sugar, milk, and vanilla until smooth. Drizzle glaze across the warm streusel. Slice into squares and serve warm.