Preheat oven to 350 degrees F°. Spray a 9×13-inch glass baking pan with cooking spray. Set aside.
FOR CAKE: In an electric stand mixer, add the flour, salt, sugar, baking powder, milk, eggs, and vanilla. Mix until just combined. Then slowly mix in melted butter. Pour batter into the prepared pan, spreading out evenly.
FOR TOPPING: In a large bowl mix the butter, brown sugar, flour and cinnamon together until well combined and creamy. Drop evenly over the batter, by the tablespoonfuls, and use a knife to marble/swirl through the cake.
Bake cake for 28-32 minutes or until a toothpick inserted in the center comes out nearly clean. Remove cake from oven and let stand at least 15 to 30 minutes before topping with glaze.
FOR GLAZE: In a medium bowl, mix the powdered sugar, milk and vanilla together with a whisk. Drizzle evenly over the warm cake. Slice and serve warm or at room temperature. Enjoy!