01 - Preheat oven to 350°F. Generously grease a 9x13-inch baking pan with butter or cooking spray.
02 - In a large mixing bowl, combine flour, granulated sugar, softened butter, milk, eggs, baking powder, salt, and vanilla extract. Beat with an electric mixer on medium speed until completely smooth and creamy, about 2-3 minutes. Pour the batter evenly into the prepared pan, spreading to corners.
03 - In a separate medium bowl, whisk together melted butter, brown sugar, cinnamon, and flour until a thick, spreadable paste forms.
04 - Drop spoonfuls of the cinnamon mixture evenly over the cake batter. Use a butter knife to gently swirl the cinnamon mixture through the batter in a circular or zigzag motion, creating a marbled effect. Avoid over-swirling to maintain distinct ribbons.
05 - Bake for 32-38 minutes until golden brown and a toothpick inserted in the center comes out clean or with moist crumbs. Begin checking at 32 minutes to prevent overbaking.
06 - While the cake bakes, whisk together powdered sugar, 2 tablespoons milk, and vanilla extract in a small bowl until smooth and pourable. Add additional milk 1 teaspoon at a time if needed to reach desired consistency.
07 - Allow the cake to cool in the pan for 10-15 minutes. Drizzle the glaze evenly over the warm cake using a spoon or piping bag. Slice into squares and serve warm for optimal flavor and texture.