Prepare the Cake Batter: In a large mixing bowl, combine the all-purpose flour, granulated sugar, baking powder, and salt. Add the milk, large eggs, vanilla extract, and melted unsalted butter. Mix until the batter is smooth and well combined.
Pour into Baking Dish: Grease a baking dish and pour the batter into it, spreading it evenly.
Create the Cinnamon Swirl: In a separate bowl, mix the softened unsalted butter, brown sugar, ground cinnamon, and all-purpose flour until well combined. The mixture should be thick and spreadable.
Add the Swirl to the Batter: Drop spoonfuls of the cinnamon mixture over the cake batter. Use a knife to swirl the cinnamon mixture through the batter, creating a marbled effect.
Bake: Preheat your oven to 350°F (175°C). Bake the cake for about 35–40 minutes, or until a toothpick inserted into the center comes out clean.
Prepare the Glaze: While the cake is baking, prepare the glaze by mixing powdered sugar, milk, and vanilla extract until smooth.
Glaze the Cake: Once the cake is done baking, remove it from the oven and immediately drizzle the glaze over the top. Allow the cake to cool slightly before serving.
