Beat 1 cup softened butter, 1 cup brown sugar, 2 tablespoons flour, and 1 tablespoon 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.
Bake in the preheated oven until a toothpick inserted into the center comes out nearly clean, 25 to 30 minutes.
Mix 2 cups confectioners’ sugar, 5 tablespoons milk, and 1 teaspoon vanilla extract in a large bowl until smooth.
Drizzle the glaze over the warm cake.
