Preheat oven to 350°F and grease a 9x13 inch baking pan.
In a large bowl, whisk together flour, sugar, baking powder, and salt.
Add milk, eggs, vanilla, and melted butter. Mix until smooth.
Pour batter into the prepared pan.
In a separate bowl, beat softened butter, brown sugar, flour, and cinnamon until fluffy.
Drop spoonfuls over batter and swirl with a knife.
Bake for 25 to 30 minutes until a toothpick comes out nearly clean.
For glaze, whisk powdered sugar, milk, and vanilla until smooth.
Drizzle glaze over warm cake and serve.