With an electric mixer or stand-up mixer, mix flour, sugar, salt, baking powder, milk, eggs, and vanilla. Once combined well, slowly stir in 4 T melted butter. Pour batter into a greased 9x13 inch baking pan.
In a large bowl, mix the 2 sticks of softened butter, brown sugar, flour, cinnamon, and nuts until well combined. Drop evenly over cake batter by the tablespoon and use a knife to marble/swirl through the cake. Bake at 350 degrees for 25-30 minutes or until toothpick comes out nearly clean from center. Place powdered sugar, milk, and vanilla in a large bowl. Whisk until smooth. Drizzle over warm cake.