Preheat the oven to 350°F (175°C). Grease a 9x13 inch baking dish.
In a large mixing bowl, whisk together the flour, sugar, baking powder, and salt.
In another bowl, combine the milk, melted butter, eggs, and vanilla extract. Mix well.
Pour the wet ingredients into the dry ingredients and stir until just combined.
Spread the batter evenly in the prepared baking dish.
In a small bowl, mix together the ground cinnamon and brown sugar. Sprinkle this mixture evenly over the batter.
Using a knife or a skewer, swirl the cinnamon mixture into the batter to create a marbled effect.
Bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean.
While the cake is baking, prepare the icing by whisking together the powdered sugar, milk, and vanilla extract until smooth.
Once the cake is done, let it cool for about 10 minutes before drizzling the icing over the top.