Preheat oven to 350°F. Grease a 9x13 baking dish.
Whisk flour, salt, sugar, and baking powder in a large bowl.
Stir in milk, eggs, vanilla, and melted butter until smooth.
Pour batter into dish.
In another bowl, mix brown sugar, flour, cinnamon, and softened butter until crumbly.
Drop spoonfuls over the batter, then swirl gently with a knife.
Bake for 35 minutes until a toothpick comes out clean.
Mix powdered sugar, milk, and vanilla for glaze.
Pour glaze over the warm cake and let it soak in a bit before serving.