Preheat oven to 350F degrees. Spray 9x13-inch baking dish with nonstick cooking spray.
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 ½ cups milk.
While your mixer is running, slowly add ½ cup (1 stick) salted butter, melted. Mix just until combined.
Spread this batter evenly into your prepared baking dish.
In a small bowl, combine ¾ 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.
Bake the cake for about 35-40 minutes, 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.
Pour the glaze over the warm cake. This cake can be served warm or at room temperature.
