FOR THE CAKE:
Preheat oven to 350F degrees. Spray 9 x 13 baking dish with nonstick cooking spray (I like the kind that has flour in it.) In a large bowl (or the bowl of your stand mixer), combine ingredients for cake (except melted butter).
While your mixer is running, slowly add the 1/2 cup melted butter. Mix just until combined (overmixing flour in baked goods can make it tough and rubbery.)
Spread this batter evenly into your baking dish.
FOR THE FILLING:
In a small bowl, combine all your filling ingredients: softened butter, brown sugar, flour and cinnamon. Stir until combined well.
Drop the cinnamon filling by spoonfuls onto the cake batter.
Take a butter knife and make swirly, marble-effect patterns all around the cake. Drag the knife through both the cinnamon and cake batter.
Bake the cake for about 35-40 minutes (it took about 40 minutes in my oven), until a knife inserted into the center comes out clean.
FOR THE GLAZE:
Whisk together powdered sugar milk and vanilla extract in small bowl.
Pour the glaze over the warm cake. yes all of it!