Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish. In a large mixing bowl, combine the flour, granulated sugar, baking powder, and salt. Add the milk, eggs, vanilla extract, and melted butter, stirring until smooth.
Step 2: Make the Cinnamon Swirl
In a separate bowl, mix the softened butter, brown sugar, flour, and cinnamon until smooth.
Step 3: Swirl the Cinnamon Mixture
Pour the cake batter into the prepared baking dish. Drop spoonfuls of the cinnamon mixture over the cake batter. Use a knife to swirl the cinnamon mixture throughout the batter to create a marbled effect.
Step 4: Bake the Cake
Bake in the preheated oven for 30-35 minutes, or until a toothpick inserted into the center comes out clean.
Step 5: Prepare the Glaze
While the cake bakes, whisk together the powdered sugar, milk, and vanilla extract until smooth. Adjust the milk for your desired consistency.
Step 6: Glaze the Cake
Once the cake is out of the oven and slightly cooled, drizzle the glaze over the top. Allow it to set for a few minutes before slicing and serving.
Notes
Swirling Technique: For a beautiful marbled effect, swirl the cinnamon mixture gently without overmixing.
Glaze Thickness: Adjust the milk amount in the glaze to achieve your preferred consistency. Thicker glaze for a frosting-like topping, thinner for a drippy finish.
Extra Flavor: Add a pinch of nutmeg to the cinnamon swirl for additional warmth and spice.