Preheat your oven to 350°F (175°C). Thoroughly grease a 9x13-inch baking pan to prevent sticking.
In a large mixing bowl, whisk together the flour, sugar, baking powder, and salt for the cake base.
In a separate bowl, combine the milk, eggs, vanilla extract, and melted butter. Whisk until smooth.
Pour the wet ingredients into the dry ingredients and mix just until combined. Be careful not to overmix - a few small lumps are okay.
Pour the batter into your prepared baking pan, spreading it evenly with a spatula.
For the cinnamon swirl, beat together the softened butter, brown sugar, flour, and cinnamon in a bowl until well combined and creamy.
Drop spoonfuls of the cinnamon mixture over the cake batter. Use a knife or skewer to swirl the cinnamon mixture throughout the batter, creating a marbled effect.
Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted in the center comes out clean with a few moist crumbs.
While the cake is baking, prepare the glaze by whisking together the powdered sugar, milk, and vanilla until smooth and pourable.
Allow the cake to cool for about 10 minutes before drizzling with the vanilla glaze.
Serve warm or at room temperature for the ultimate cinnamon cake experience.