For the cake
1-Preheat oven to 350F degrees: Preheat oven to 350F degrees. Spray 9×13-inch baking dish with nonstick cooking spray.
2-In a large bowl (or the bowl: 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.
3-While your mixer is running, slowly add: While your mixer is running, slowly add 1/2 cup (1 stick) salted butter, melted. Mix just until combined.
4-Spread this batter evenly into your prepared: Spread this batter evenly into your prepared baking dish.

For the filling
5-In a small bowl, combine 3/4 cup: 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.
6-Drop the cinnamon filling by spoonfuls onto: Drop the cinnamon filling by spoonfuls onto the cake batter.
7-Take a butter knife and make swirly,: Take a butter knife and make swirly, marble-effect patterns all around the cake. Drag the knife through both the cinnamon and cake batter.
8-Bake the cake for about 35-40 minutes: 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
9-Whisk together 2 cups powdered sugar, 5: Whisk together 2 cups powdered sugar, 5 Tablespoons milk and 1 teaspoon vanilla extract in a bowl.
10-Pour the glaze over the warm cake: Pour the glaze over the warm cake.