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 with electric mixer until smooth and creamy, approximately 2-3 minutes.
03 - Pour the cake batter evenly into the prepared baking pan, spreading to the corners with a spatula.
04 - In a separate bowl, mix melted butter, brown sugar, cinnamon, and flour until a thick, spreadable paste forms.
05 - Drop spoonfuls of the cinnamon mixture evenly over the batter. Use a knife to gently swirl the cinnamon mixture through the batter, creating a marbled effect without over-mixing.
06 - Bake for 32-38 minutes until golden brown and a toothpick inserted in the center comes out clean.
07 - While cake bakes, whisk together powdered sugar, 2-3 tablespoons milk, and vanilla extract until smooth and pourable. Add additional milk if needed to reach desired consistency.
08 - Allow cake to cool for 10-15 minutes. Drizzle glaze evenly over the warm cake. Slice into squares and serve warm.