01 - Preheat oven to 350°F. Grease a 9x13-inch baking pan thoroughly with butter or cooking spray.
02 - In a large bowl, combine flour, granulated sugar, softened butter, milk, eggs, baking powder, salt, and vanilla extract. Beat until smooth and creamy, about 2-3 minutes. Pour batter evenly into prepared pan.
03 - In a separate bowl, whisk together melted butter, brown sugar, cinnamon, and flour until thick paste forms.
04 - Drop spoonfuls of cinnamon mixture over cake batter. Using a knife, swirl through batter creating marble effect, being careful not to over-mix.
05 - Bake for 32-38 minutes until toothpick inserted in center comes out clean. Cake should be golden brown.
06 - While cake bakes, whisk powdered sugar, 2-3 tablespoons milk, and vanilla until smooth and pourable. Add more milk if needed.
07 - Let cake cool 10-15 minutes. Drizzle glaze evenly over warm cake. Slice and serve warm.