Preheat the oven to 400 degrees . Spray a parchment lined 9-inch round cake pan with unflavored cooking spray. Melt 1 stick of the butter and set aside.
In a small saucepan, melt the remaining 4 tablespoons of butter with the brown sugar over medium heat, about 3 minutes. Spread the butter-and-sugar mixture in the bottom of the prepared cake pan. Distribute the pineapple slices evenly over the mixture and set the pan aside.
In a large bowl, combine the flour, baking powder, sugar and salt. In a medium bowl, beat together the egg, milk and the reserved melted butter. Stir the liquid ingredients into the dry ingredients until well combined; the batter will be thick. Using a rubber spatula, scoop the batter into the cake pan, spreading it until it reaches the sides of the pan.
Bake the cake until the top is golden brown and a toothpick inserted in the center comes out clean, about 30 to 35 minutes.
Let the cake cool for 15 minutes. Place a serving plate over the cake pan and invert. Slowly lift off the pan and serve.