Preheat oven to 350 degrees. With an electric mixer or stand mixer, combine the flour, sugar, salt, baking powder, milk, eggs, and vanilla. Once that is combined, you slowly stir in 4 T melted butter. Pour the batter into a greased 9×13 inch pan.
In a medium sized bowl, mix the softened butter, brown sugar, cinnamon and nuts if you are using them. Drop evenly over the cake better in your 9×13 inch pan. Use a knife to marble and swirl over the cake.
Bake for 25-30 minutes or until toothpick comes out nearly clean from center.
Make the glaze by adding the powdered sugar, milk and vanilla to a medium sized bowl. Whisk until smooth and drizzle over warm cake. Serve warm or at room temperature.