01 - Preheat oven to 350°F. Grease a 9x13-inch baking pan thoroughly with butter or cooking spray.
02 - In a large bowl, beat flour, granulated sugar, softened butter, milk, eggs, baking powder, salt, and vanilla extract until smooth and creamy. Pour batter evenly into the prepared pan.
03 - In a separate bowl, combine melted butter, brown sugar, cinnamon, and flour. Mix until a thick, spreadable paste forms.
04 - Drop spoonfuls of cinnamon mixture over the cake batter. Use a knife to gently swirl the cinnamon through the batter, creating a marbled ribbon effect.
05 - Bake for 32-38 minutes until a toothpick inserted in the center comes out clean.
06 - While cake bakes, whisk powdered sugar, milk, and vanilla until smooth and pourable.
07 - Cool cake for 10-15 minutes, then drizzle glaze evenly over warm cake. Slice and serve immediately.