Preheat your oven to 350°F (175°C) and grease a 9x13 inch baking dish.

In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.

In a large mixing bowl, cream together the softened butter and granulated sugar using an electric mixer for 2-3 minutes until light and fluffy.

Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.

Gradually add the dry ingredients to the wet mixture, alternating with the milk, mixing until just combined.

In a small bowl, combine the brown sugar and ground cinnamon. Stir in the melted butter until well mixed.

Pour half of the cake batter into the prepared baking dish and spread it evenly.

Sprinkle the cinnamon mixture over the batter, then pour the remaining batter on top and spread gently.

Use a knife or skewer to gently swirl the cinnamon mixture into the cake batter.

Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.

Let the cake cool for about 10 minutes before preparing the glaze.

In a small bowl, whisk together the powdered sugar, milk, and vanilla extract until smooth.

Drizzle the glaze over the warm cake and enjoy!