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

In a large bowl, whisk together flour, sugar, baking powder, and salt.

In a separate bowl, combine milk, eggs, and melted butter until smooth.

Gently mix wet ingredients into dry ingredients until just combined.

In a small bowl, mix brown sugar, melted butter, and cinnamon for the swirl.

Layer half the batter in the baking dish, sprinkle with half the cinnamon mix, add remaining batter, and top with remaining cinnamon mix.

Use a knife or toothpick to swirl through the batter gently.

Bake in the preheated oven for 25-30 minutes or until golden.

Let it cool for 10 minutes in the pan before transferring to a wire rack.

Prepare glaze by whisking powdered sugar, milk, and vanilla together until smooth.

Once cooled, drizzle glaze over the cake, slice, and serve.