Whisk together flour, brown sugar, cinnamon, pecans, and salt in a medium bowl.

Add melted butter and mix with a fork until large crumbs form. Set aside.

Stir together brown sugar and cinnamon in a small bowl. Set aside.

Preheat oven to 350°F (175°C). Grease and line a 9x13-inch pan with parchment paper, leaving an overhang for lifting later.

In a medium bowl, whisk flour, baking powder, baking soda, and salt.

In a large bowl, cream butter and sugar with an electric mixer until light and fluffy, about 3 minutes.

Beat in eggs one at a time, then add vanilla.

Mix in sour cream until smooth.

With mixer on low, alternate adding the flour mixture and milk in three additions, starting and ending with flour. Do not overmix.

Spread half of the batter evenly into the prepared pan.

Sprinkle the cinnamon-sugar filling evenly over the batter.

Gently spread the remaining batter on top.

Sprinkle streusel evenly over the batter, pressing lightly so it adheres.

Bake 45–55 minutes, or until a toothpick inserted in the center comes out clean. If the streusel browns too quickly, tent loosely with foil.

Cool completely in the pan on a wire rack.

Whisk powdered sugar, milk, and vanilla until smooth.

Drizzle over the cooled cake before slicing into squares.