Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking pan with butter to prevent sticking.
In a large mixing bowl, combine 3 cups of all-purpose flour, 1/4 tsp salt, 1 cup of sugar, and 4 tsp of baking powder. Mix until the dry ingredients are well integrated.
Add 1 1/2 cups of milk, 2 large eggs, 2 tsp of vanilla extract, and 1/2 cup of melted butter. Stir until the batter is smooth and evenly combined.
Pour the batter into the prepared baking pan, spreading it evenly with a spatula.
In a separate bowl, prepare the topping. Combine 1 cup of softened butter, 1 cup of brown sugar, 2 tbsp of flour, and 1 tbsp of cinnamon. Mix until it forms a thick paste.
Drop spoonfuls of the topping over the batter, spacing them evenly. Using a butter knife, gently swirl the topping into the batter to create a marbled effect.
Bake for 28-30 minutes, or until a toothpick inserted in the center comes out clean. The cake should be golden brown and spring back when lightly touched.
While the cake is baking, prepare the glaze. In a medium bowl, whisk together 2 cups of powdered sugar, 5 tbsp of milk, and 1 tsp of vanilla until smooth.
Once the cake is done baking, remove it from the oven and allow it to cool for a few minutes. Drizzle the warm glaze over the cake before serving.