Grease a 9"x13" cake pan (or line it with parchment paper or aluminum foil and grease the sides and bottom). Set aside.
Preheat the oven to 350 degrees F.
In a large mixing bowl, whisk together flour, salt, sugar and baking powder.
Add milk, eggs and vanilla and whisk until smooth.
Slowly add the melted butter and whisk until everything is smooth.
Spread the batter in the prepared pan. Set aside.
In a medium mixing bowl, beat butter with electric mixer until creamy.
Add sugar, flour and cinnamon and beat until fluffy, about 2 minutes. (It will look crumbly at first but keep beating, it will come together.)
Spread the filling over the cake batter, leaving at least 1 inch of space from the edge of the pan. Swirl gently with a off-set icing spatula or a rubber spatula.
Bake the cake for 28 to 32 minutes or until the toothpick inserted in the cake part comes out clean.
Cool the cake for 15 minutes. Pour the glaze over the cake and spread evenly.