Preheat oven to 350°F (175°C).
Grease your 9x13 inch baking pan thoroughly.
Mix flour, sugar, baking powder, and salt together.
Whisk eggs, milk, melted butter, and vanilla separately.
Combine wet and dry ingredients until just mixed.
Pour half the batter into prepared pan.
Mix brown sugar, cinnamon, and melted butter together.
Sprinkle cinnamon mixture over the first batter layer.
Add remaining batter on top of cinnamon layer.
Swirl gently with knife for marbled effect.
Bake for 30-35 minutes until golden brown.
Test doneness with toothpick in center area.
Cool completely before adding the sweet glaze.
Whisk powdered sugar, milk, and vanilla together.
Drizzle glaze over cooled cake before serving.