Preheat your oven to 350°F (175°C). Grease the 9x13 inch baking pan.
In a large mixing bowl, whisk together the flour, granulated sugar, baking powder, and salt.
Add the softened butter, milk, eggs, and vanilla extract to the dry ingredients. Mix until just combined and smooth.
In a separate bowl, mix the brown sugar and cinnamon for the filling.
Pour half of the cake batter into the prepared baking pan, spreading it evenly. Sprinkle the cinnamon filling mixture over the batter, then pour the remaining batter on top.
Using a knife, gently swirl through the batter to create a marbled effect.
Bake in the preheated oven for 30-40 minutes, or until a toothpick inserted in the center comes out clean.
While the cake is baking, prepare the glaze by whisking together powdered sugar, milk, and vanilla extract in a small saucepan over low heat until smooth.
Once the cake is done, remove it from the oven and let it cool for about 10 minutes. Drizzle the warm glaze over the top.
Allow the cake to cool completely in the pan on a cooling rack. Cut into squares and serve.