Preheat the oven to 350°F (175°C). Grease a 9×13-inch baking pan and set aside.
In a medium bowl, whisk together flour, baking powder, and salt. Set dry mix aside.
In a large bowl, cream the softened butter and granulated sugar until light and fluffy.
Add eggs one at a time, mixing well after each addition. Stir in milk and vanilla extract.
Gradually add dry ingredients to wet, mixing until just combined to form a smooth batter.
Pour half of the batter into the prepared pan. Smooth surface with a spatula.
In a small bowl, combine brown sugar, ground cinnamon, and melted butter. Sprinkle evenly over batter.
Top swirl with remaining batter, gently spreading to cover cinnamon sugar layer.
Bake in preheated oven for 22–25 minutes, or until a toothpick inserted in the center comes out clean.
While cake bakes, whisk together powdered sugar, milk, and vanilla extract to make glaze.
Let cake cool for 10 minutes, then drizzle glaze evenly over warm cake.
Slice into squares and serve warm or at room temperature.