Preheat your oven to 350°F and line a square 8x8 baking dish with parchment paper (or spray with nonstick cooking spray).
Whisk together the flour, baking soda, and baking powder in a medium-sized bowl. Set aside.
In a larger mixing bowl, whisk together the butter and sugar until creamy and light (about 2-3 minutes).
Add in an egg and the vanilla. Mix in the sour cream.
Add dry ingredients to butter mixture and mix just until combined.
To make the streusel topping, whisk together the flour, brown sugar, salt, and cinnamon. Add in the melted butter and stir until it's the texture of sand. Add in the chopped pecans.
Spread half of the cake batter into the prepared pan. Sprinkle with half of the streusel. Dollop the remaining batter on top and smooth into an even layer. Sprinkle the remaining streusel over the top.
Bake for 35-45 minutes until golden brown on top and a toothpick inserted into the center of the cake comes out clean or with few crumbs.
Allow to cool for at least 10-15 minutes, then cut into portions.