Preheat oven to 350F degrees. Spray a 9x13-inch baking dish with nonstick cooking spray. In a large bowl or a stand mixer, combine 3 cups all-purpose flour, 1 cup white granulated sugar, 4 teaspoons baking powder, 2 large eggs, 2 teaspoons vanilla extract, and 1 ½ cups milk.
Slowly add the melted ½ cup salted butter. Mix until combined. Do not over-mix. Spread this batter evenly into your prepared baking dish.
For The Filling:
In a small bowl, combine 3/4 cup salted butter, softened to room temperature, 1 cup packed light brown sugar, 2 tablespoons all-purpose flour and 1 tablespoon ground cinnamon. Stir until combined well.
Drop the cinnamon filling by a spoonful onto the cake batter.
Take a butter knife and swirl the batter until it’s marbled.
Bake the cake for about 35-40 minutes, until the center is fully cooked.
For The Glaze:
Whisk together 2 cups powdered sugar, 5 tablespoons milk and 1 teaspoon vanilla extract in a bowl. Pour the glaze over the warm cake.