Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking dish.
In a large mixing bowl, whisk together the flour, granulated sugar, baking powder, and salt.
In another bowl, combine the milk, melted butter, eggs, and vanilla extract.
Pour the wet ingredients into the dry ingredients and mix until just combined.
In a small bowl, mix the ground cinnamon and brown sugar together.
Pour half of the cake batter into the prepared baking dish and spread it evenly.
Sprinkle half of the cinnamon-sugar mixture over the batter.
Pour the remaining batter on top and finish with the rest of the cinnamon-sugar mixture.
Use a knife to swirl the cinnamon mixture into the batter gently.
Bake for 30-35 minutes or until a toothpick inserted in the center comes out clean.
While the cake is baking, prepare the glaze by whisking together the powdered sugar, milk, and vanilla extract in a small bowl until smooth.
Once the cake is done, let it cool for about 10 minutes in the pan before drizzling the glaze over the top.