Pour into the greased baking dish or pan and set aside.
For the topping layer, stir together melted butter, brown sugar, cinnamon, flour and chopped walnuts (if using) in a small mixing bowl until smooth. This will look thin, but don’t worry---this is the delicious part!
Spread this topping over the batter in your baking dish. Make sure it goes from corner to corner, then swirl it a few times with a knife to give it a marbled look. Don’t overdo it or the cake will look layered instead of marbled. Don’t forget to swirl into the corners or the edges if you’re using the round pan.
Bake for 30-35 minutes or until a toothpick inserted in the middle comes out clean.
While the cake is baking, make your glaze by stirring together in a small bowl the powdered sugar, milk, and vanilla extract until all lumps have disappeared. Set this aside until the cake is done.
Immediately after removing the cake from the oven drizzle the glaze over the lightly browned top of your cake. This will seal your cake and give it a lovely sheen.