Preheat oven to 350°F. Grease a 9x13 inch baking pan with nonstick spray or butter.
In a large bowl, whisk together 3 cups flour, 1 cup granulated sugar, 4 teaspoons baking powder, and 1/4 teaspoon salt.
Add 1 1/2 cups milk, 2 eggs, and 2 teaspoons vanilla. Mix until smooth. Slowly stir in 4 tablespoons melted butter.
Pour batter into prepared pan and spread evenly. Set aside.
For filling, beat 1/2 cup softened butter, 1 cup brown sugar, 2 tablespoons flour, and 1 tablespoon cinnamon until thick paste forms.
Drop filling by tablespoonfuls across batter. Use butter knife to swirl gently 3-4 times, creating marbled effect.
Bake for 35-40 minutes until toothpick inserted in center comes out clean. Top should be golden brown.
Cool for 20 minutes. Whisk together 2 cups powdered sugar, 5 tablespoons milk, and 1 teaspoon vanilla until smooth.
Drizzle glaze over warm cake. Serve warm or at room temperature.