Preheat the oven to 350°F (175°C) and grease a 9x13-inch baking pan.
In a large bowl, mix 3 cups of flour, milk, white sugar, eggs, baking powder, 2 teaspoons of vanilla extract, and salt until smooth.
Stir in ¼ cup of melted butter.
Pour the batter evenly into the greased baking pan.
In a separate bowl, beat 1 cup of softened butter, brown sugar, 2 tablespoons of flour, and cinnamon until smooth and well combined.
Drop spoonfuls of the cinnamon mixture over the batter in the pan.
Use a knife to swirl the cinnamon mixture into the batter, creating a marbled effect.
Bake for 25 to 30 minutes, or until a toothpick inserted into the center comes out almost clean.
While the cake is baking, whisk together confectioners' sugar, milk, and 1 teaspoon of vanilla extract in a bowl until smooth.
Drizzle the glaze over the warm cake before serving.