1. Prepare the batter:
- Preheat your oven to 350°F (175°C). Grease a 9×13-inch baking dish.
- In a large mixing bowl, whisk together the flour, sugar, baking powder, and salt.
- Add the milk, eggs, vanilla extract, and melted butter. Stir until just combined (do not overmix). Pour the batter into the prepared baking dish and spread evenly.
2. Make the cinnamon swirl:
- In a small bowl, combine the brown sugar, cinnamon, and softened butter until smooth.
- Drop spoonfuls of the cinnamon mixture over the batter, then use a knife to swirl it into the batter to create a marbled effect.
3. Bake the cake:
- Bake in the preheated oven for 30–35 minutes, or until a toothpick inserted in the center comes out clean.
4. Prepare the glaze:
- In a medium bowl, whisk together the powdered sugar, milk, and vanilla extract until smooth. Adjust the consistency by adding more milk if needed.
5. Glaze and serve:
- While the cake is still warm, pour the glaze over the top. Spread it evenly to cover the entire surface.
- Let the cake cool slightly before slicing and serving.