Step 1

Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking pan.

Step 2

Mix 3 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.

Step 3

Beat 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.

Step 4

Bake in the preheated oven until a toothpick inserted into the center comes out nearly clean, 25 to 30 minutes.

Step 5

Mix confectioners' sugar, milk, and 1 teaspoon vanilla extract in a large bowl until smooth. Drizzle over warm cake.