Preheat oven to 350°F (175°C) and grease a 9x13-inch baking dish.
In a large bowl, mix flour, sugar, baking powder, and salt.
Add milk, eggs, vanilla extract, and melted butter. Stir until smooth.
Pour the batter evenly into the prepared baking dish.
In a separate bowl, mix softened butter, brown sugar, flour, and cinnamon.
Drop spoonfuls of the cinnamon mixture over the batter.
Use a knife to swirl the mixture into the batter.
Bake for 35–40 minutes or until a toothpick inserted in the center comes out clean.
In a bowl, whisk together powdered sugar, milk, and vanilla to make the glaze.
Drizzle the glaze over the warm cake before serving.