Preheat your oven to 350°F (175°C). Grease a 9x13 inch baking pan.
In a large mixing bowl, combine the flour, sugar, baking powder, and salt. Add the milk, eggs, vanilla extract, and melted butter. Mix until smooth.
Pour the batter into the prepared baking pan.
In a separate bowl, mix the softened butter, brown sugar, flour, and cinnamon for the cinnamon swirl. Drop spoonfuls of the mixture over the cake batter and use a knife to swirl it through the batter.
Bake in the preheated oven for 35-40 minutes, or until a toothpick inserted into the center comes out clean.
While the cake is baking, prepare the glaze by mixing the powdered sugar, milk, and vanilla extract until smooth.
Once the cake is done, remove it from the oven and let it cool for 10-15 minutes. Drizzle the glaze over the warm cake.