Preheat the oven to 350°F.

Grease and line a 9x13 inch pan with a piece of parchment paper. Set aside.

In a large bowl, combine the cake mix, eggs, vanilla extract, and sour cream with a wooden spoon or a whisk. Stir until all of the ingredients are well combined. Then set aside.

In a medium bowl, add the flour, melted butter, salt (if using), cinnamon, brown sugar, and pecans. Using a fork, pastry cutter, or spoon, stir the ingredients together until they're all well combined. Set aside.

Add half of the cake batter to the prepared 9x13 inch pan.

Add half of the streusel mixture over the top of the cake batter.

Spread the remaining cake batter over the top of the streusel layer.

Finally, sprinkle the remaining streusel over the top.

Bake at 350℉ for 45 minutes or until a toothpick comes out clean in the center.

Allow the cake to cool for 45 minutes before serving.

Once the cake has finished baking and it's cooling, in a bowl combine the powdered sugar, milk, and vanilla. Using a fork or spoon, stir to combine.

Drizzle the glaze over top and serve.