Preheat your oven to 350°F (175°C). Grease a 9-inch round cake pan with butter or cooking spray.
In a small bowl, combine the melted butter and brown sugar. Mix well until the sugar is fully dissolved. Pour the mixture into the prepared cake pan, spreading it evenly to cover the bottom.
Arrange the pineapple slices on top of the brown sugar mixture in a single layer. You can also add maraschino cherries in the center of each pineapple ring, if desired.
In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.
In a large bowl, cream together the softened butter and granulated sugar until light and fluffy. This can be done using an electric mixer or by hand using a whisk.
Add the eggs, one at a time, beating well after each addition. Stir in the vanilla extract.
Gradually add the flour mixture to the butter mixture, alternating with the pineapple juice and milk. Begin and end with the flour mixture, mixing until just combined. Be careful not to overmix the batter.
Pour the batter over the pineapple slices in the cake pan, spreading it evenly with a spatula.
Bake for 40-45 minutes, or until a toothpick inserted into the center of the cake comes out clean.
Remove the cake from the oven and let it cool in the pan for 10 minutes. Place a serving plate upside down on top of the cake pan, then carefully flip the pan over to release the cake onto the plate. Gently lift the pan, revealing the beautiful pineapple topping.
Allow the cake to cool completely before serving. You can serve it warm or at room temperature, depending on your preference.