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 or whisk until smooth and creamy, about 2-3 minutes. Pour batter evenly into prepared pan.
03 - In a separate medium bowl, whisk together melted butter, brown sugar, cinnamon, and flour until a thick, spreadable paste forms.
04 - Drop spoonfuls of cinnamon mixture evenly over cake batter. Using a butter knife, gently swirl through the batter in circular or figure-eight motions to create marbled effect. Avoid over-mixing.
05 - Bake for 32-38 minutes or until golden brown and a toothpick inserted in center comes out clean. Rotate pan halfway through baking for even cooking.
06 - While cake bakes, whisk powdered sugar, 2 tablespoons milk, and vanilla extract in small bowl until smooth and pourable. Add additional milk if needed to reach desired consistency.
07 - Remove cake from oven and let cool in pan for 10-15 minutes. Drizzle glaze evenly over warm cake. Slice into squares and serve warm for optimal flavor and texture.