01 - Preheat oven to 350°F. Generously grease a 9x13-inch baking pan 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 on medium speed until smooth and creamy, approximately 2-3 minutes.
03 - Spread cake batter evenly into the prepared baking pan, smoothing the top with a spatula.
04 - In a separate medium bowl, whisk together melted butter, brown sugar, cinnamon, and flour until a thick, spreadable paste forms.
05 - Drop spoonfuls of the cinnamon mixture evenly over the cake batter. Use a butter knife to gently swirl the cinnamon mixture through the batter in a figure-eight motion, creating a marbled pattern.
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 tablespoons milk, and vanilla extract in a small bowl until smooth. Add additional milk, 1 teaspoon at a time, if needed to achieve pourable consistency.
08 - Allow cake to cool in pan for 10-15 minutes. Drizzle glaze evenly over the warm cake. Slice into squares and serve warm.