01 - Preheat oven to 350°F. Grease a 9x13-inch baking pan thoroughly with butter or cooking spray.
02 - In a large mixing bowl, beat flour, granulated sugar, softened butter, milk, eggs, baking powder, salt, and vanilla extract until completely smooth and creamy, approximately 2-3 minutes.
03 - Pour the prepared batter evenly into the greased baking pan, spreading to corners with a spatula.
04 - In a separate medium bowl, combine melted butter, brown sugar, cinnamon, and flour. Mix vigorously until a thick, spreadable paste forms.
05 - Drop spoonfuls of cinnamon mixture evenly over the cake batter. Using a knife, gently swirl the cinnamon mixture through the batter in circular or zigzag motions to create a marbled effect.
06 - Place pan in preheated oven and bake for 32-38 minutes, or until a toothpick inserted into the center emerges clean or with moist crumbs.
07 - While cake bakes, whisk powdered sugar, milk, and vanilla extract in a small bowl until smooth and pourable. Adjust consistency with additional milk if needed.
08 - Remove cake from oven and let cool in pan for 10-15 minutes. Drizzle glaze evenly over warm cake. Slice into squares and serve immediately for optimal flavor and texture.