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

In a large mixing bowl, combine the flour, granulated sugar, baking powder, and salt; whisk until well blended.

Pour in the melted butter, milk, eggs, and vanilla extract; stir until just combined.

In a separate bowl, mix the ground cinnamon with brown sugar to create the cinnamon filling.

Pour half of the batter into the prepared baking pan, sprinkle half of the cinnamon sugar mixture on top, then pour the remaining batter over it and finish with the rest of the cinnamon sugar mixture.

Using a knife, gently swirl through the batter to create a marbled effect.

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

Let the cake cool in the pan for about 10 minutes, then transfer it to a wire rack.

For the frosting, mix powdered sugar with 2-3 tablespoons of milk and drizzle it over the cooled cake before serving.