01 - Preheat oven to 350°F. Grease a 9x13-inch baking pan thoroughly.
02 - In a large bowl, beat together 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, mix melted butter, brown sugar, cinnamon, and flour until a thick, spreadable paste forms.
04 - Drop spoonfuls of the cinnamon mixture over the cake batter. Use a knife to swirl the cinnamon mixture through the batter, creating a marbled effect.
05 - Bake for 32-38 minutes, or until a toothpick inserted in the center comes out clean.
06 - While cake is baking, whisk together powdered sugar, milk, and vanilla extract until smooth and pourable.
07 - Allow cake to cool for 10-15 minutes, then drizzle glaze evenly over the warm cake. Slice and serve warm for best flavor.