Preheat the oven to 350°F.
Drain the pineapple slices; save ⅓ cup of the juice for the cake.
Spray the bottom and sides of a 9-inch round cake pan (preferably nonstick) with cooking spray.
Pour the melted butter into the cake pan; swirl it around so that it coats the bottom of the pan.
Sprinkle the brown sugar evenly over the butter.
In a large bowl, cream together the softened butter and the granulated sugar until well-combined, light, and fluffy.
Add the egg and vanilla; beat until well-combined and the mixture is smooth.
Add the milk and reserved pineapple juice; stir until well-combined. Note: The mixture will look a little curdled/broken at this point. It's totally normal!
In a small bowl, whisk together the flour, baking powder, and salt.
Add the flour mixture to the wet ingredients; stir just until the flour is completely combined and the mixture is smooth (don't over-mix the batter, as it can cause the cake to be dense and tough).
Gently spoon the batter into the pan and smooth it out, being very careful not to disturb the arrangement of the pineapple and cherries.
Bake in the center of the oven for 30 to 35 minutes or until a wooden toothpick inserted in the center of the cake comes out clean or with a few moist crumbs (no wet batter). Note: Be careful not to over-bake the cake, as this can make it crumbly.
Place the pan on a wire cooling rack and allow to cool for 5 minutes. Note: Don't wait any longer to flip the cake; the cooler it gets, the more it will be prone to sticking.
Run a knife along the edges of the cake to loosen it from the pan.
Place your serving dish over the cake. Using oven mitts or something else to protect your hands, carefully invert the cake pan onto the serving dish. Remove the cake pan (you can give it a gentle wiggle or lightly tap the pan with a wooden spoon to help it along, but it should come out easily). If any pieces of fruit have stuck to the pan, just remove them and put them back on the cake.
Allow the cake to cool either until it is still slightly warm or until it has completely cooled to room temperature.