Heat your oven to 350°F (175°C). Lightly grease a 9×13-inch baking dish using butter or a nonstick cooking spray so the cake releases easily after baking.

Add the flour, granulated sugar, baking powder, and salt to a large mixing bowl and stir them together. Crack in the eggs, then pour in the milk, vanilla extract, and melted butter. Stir just until the batter looks smooth and no dry flour is left. Pour the batter into the prepared baking dish and spread it into an even layer.

Place the softened butter, brown sugar, flour, and ground cinnamon into another bowl. Stir until the mixture becomes smooth, thick, and creamy without any dry spots.

Spoon small portions of the cinnamon mixture over the top of the cake batter, spreading them across the surface so every slice gets plenty of cinnamon flavor.

Take a butter knife or wooden skewer and gently drag it through the batter in loose swirling motions. A few passes are enough. Leave visible streaks of cinnamon instead of blending everything together.

Transfer the dish to the hot oven and bake for 30 to 35 minutes. Check for doneness by inserting a toothpick into the center. If it comes out clean or with only a few moist crumbs, the cake is ready.

While the cake is baking, whisk together the powdered sugar, milk, and vanilla extract until silky smooth. If the glaze feels too thick, stir in a small splash of milk until it reaches a pourable consistency.

Let the cake rest for about 10 minutes after removing it from the oven. Pour the vanilla glaze over the warm cake so it melts slightly into the top. Cut into 12 slices and enjoy while still warm for the richest cinnamon roll taste.