Preheat oven to 350F degrees. Spray 9x13-inch 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 3 cups all-purpose flour, 1 cup white granulated sugar, 4 teaspoons baking powder, 2 large eggs, 2 teaspoons vanilla extract and 1 1/2 cups milk.

While your mixer is running, slowly add 1/2 cup (1 stick) salted butter, melted. Mix just until combined (overmixing flour in baked goods can make it tough and rubbery.)

Spread this batter evenly into your prepared baking dish.

In a small bowl, combine 3/4 cup salted butter, softened to room temperature, 1 cup packed light brown sugar, 2 Tablespoons all-purpose flour and 1 Tablespoon ground 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.

Whisk together 2 cups powdered sugar, 5 Tablespoons milk and 1 teaspoon vanilla extract in a bowl. If you want it thicker, add more powdered sugar and if you want it thinner, add more milk.

Pour the glaze over the warm cake. This cake can be served warm or at room temperature but I think it's easier to slice if you allow it to cool first.