1Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking pan.
2Mix 2 cups flour, milk, white sugar, eggs, baking powder, 2 teaspoons vanilla extract, and salt together in a large bowl until well-combined. Stir in 1/4 cup melted butter. Pour batter into the prepared baking pan.
3Beat 1 cup softened butter, brown sugar, 2 tablespoons flour, and cinnamon together in a large bowl until smooth. Drop by spoonfuls over the batter in the baking pan. Swirl the top layer into the bottom layer with a knife to create a marble effect.
4Bake in the preheated oven until a toothpick inserted into the center comes out nearly clean, 25 to 30 minutes.
5Mix confectioners' sugar, milk, and 1 teaspoon vanilla extract in a large bowl until smooth. Drizzle over warm cake.
Footnotes
Tip
Aluminum foil helps keep food moist, ensures it cooks evenly, keeps leftovers fresh, and makes clean-up easy.