01 - Preheat oven to 350°F. Grease a 9x13-inch baking pan thoroughly 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 electric mixer on medium speed until smooth and creamy, approximately 2-3 minutes. Pour batter evenly into the prepared baking pan.
03 - In a separate medium bowl, whisk together melted butter, brown sugar, cinnamon, and flour until a thick, spreadable paste forms. Ensure all ingredients are fully incorporated.
04 - Drop spoonfuls of the cinnamon mixture evenly over the surface of the cake batter. Use a butter knife or skewer to gently swirl the cinnamon mixture through the batter, creating a marbled pattern. Be careful not to overmix.
05 - Bake for 32-38 minutes, or until a toothpick inserted in the center comes out clean or with just a few moist crumbs. The top should be golden brown and slightly springy to the touch.
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 cake to cool in the pan for 10-15 minutes. Drizzle the glaze evenly over the warm cake. Slice into squares and serve warm for optimal flavor and texture.