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

In a large bowl, mix the flour, granulated sugar, baking powder, and salt until well combined.

In another bowl, whisk together the milk, melted butter, eggs, and vanilla until the mixture is smooth.

Pour the wet ingredients into the dry ingredients and gently stir until just combined, avoiding over-mixing.

In a small bowl, mix the brown sugar and ground cinnamon for the filling.

Spoon half of the batter into the prepared baking dish. Sprinkle half of the cinnamon sugar mixture on top, then layer the remaining batter.

Gently swirl the batter with a knife to create a marbled effect.

Bake for 25-30 minutes, until a toothpick inserted in the center comes out clean.

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

Drizzle the glaze over the warm cake before serving to ensure it seeps into the crevices.