Prepare the Cake Batter:
Preheat the oven to 350°F (175°C). Grease a 9x13-inch baking dish.
In a large bowl, combine the flour, salt, sugar, and baking powder.
In another bowl, whisk together the milk, eggs, and vanilla extract.
Gradually mix the wet ingredients into the dry ingredients. Add the melted butter and stir until smooth. Pour the batter into the prepared baking dish.
Prepare the Cinnamon Swirl:
In a medium bowl, mix the softened butter, brown sugar, flour, and cinnamon until well combined and creamy.
Drop spoonfuls of the cinnamon mixture evenly over the cake batter. Use a knife to swirl the cinnamon mixture into the batter, creating a marbled effect.
Bake the Cake:
Bake for 30-35 minutes or until a toothpick inserted into the center comes out clean. Remove from the oven and let cool slightly.
Prepare the Glaze:
In a small bowl, whisk together powdered sugar, milk, and vanilla extract until smooth. Adjust the consistency by adding more milk if needed.
Finish and Serve:
Drizzle the glaze evenly over the warm cake. Allow the glaze to set for a few minutes before slicing and serving.