Preheat the oven to 350°F (175°C) and grease a 9x13-inch baking pan.

For the white cake layer: In a large bowl, combine 3 cups of flour, milk, white sugar, eggs, baking powder, 2 teaspoons of vanilla extract, and salt. Stir until the mixture is smooth and well-blended. Add in the ¼ cup of melted butter and mix until fully incorporated. Pour the batter into the prepared baking pan.

For the cinnamon layer: In a separate large bowl, beat together 1 cup of softened butter, brown sugar, 2 tablespoons of flour, and cinnamon until smooth. Drop spoonfuls of the cinnamon mixture evenly over the batter in the baking pan.

Using a knife, gently swirl the cinnamon mixture into the cake batter to create a marbled effect.

Bake the cake in the preheated oven for 25 to 30 minutes, or until a toothpick inserted into the center comes out nearly clean.

For the glaze: In a large bowl, whisk together the confectioners' sugar, milk, and 1 teaspoon of vanilla extract until smooth.

Once the cake is done, drizzle the glaze over the warm cake. Serve and enjoy!