Preheat the oven to 350°F and spray a 9×13-inch baking dish with nonstick spray (preferably one with flour).
In a large bowl or a stand mixer, combine 3 cups of all-purpose flour, 1 cup of granulated sugar, 4 teaspoons of baking powder, 2 large eggs, 2 teaspoons of vanilla extract, and 1½ cups of milk.
With the mixer running, slowly pour in ½ cup of melted salted butter and mix just until everything is combined, being careful not to overmix.
Spread the batter evenly into the prepared baking dish.
In a small bowl, mix ¾ cup softened salted butter, 1 cup packed light brown sugar, 2 tablespoons all-purpose flour, and 1 tablespoon ground cinnamon until well combined.
Drop spoonfuls of this cinnamon filling evenly over the cake batter.
Using a butter knife, swirl the filling through the batter to create a marble effect.
Bake for about 35–40 minutes or until a knife inserted into the center comes out clean.
While the cake is baking, whisk together 2 cups powdered sugar, 5 tablespoons milk, and 1 teaspoon vanilla extract to make the glaze, adjusting the consistency with more sugar or milk as needed.
Once the cake is done, pour the glaze over the warm cake and allow it to cool slightly before slicing and serving.