Preheat oven to 350°F (175°C). Grease the sides of two 9-inch round cake pans.
Pour the 1/2 cup melted butter evenly into the bottom of one cake pan. Sprinkle the brown sugar evenly over the butter.
Pat the pineapple slices and cherries dry. Arrange the pineapple rings over the brown sugar in a single layer. Place a cherry in the center of each ring.
In a medium bowl, whisk together the flour, baking powder, and salt.
In a large bowl, beat the softened butter and granulated sugar until light and fluffy (about 3 mins).
Add the eggs one at a time, beating well. Stir in the vanilla extract.
Combine the milk and reserved pineapple juice. Alternate adding the dry ingredients and the milk mixture to the butter, mixing just until combined.
Divide the batter evenly between the two pans, carefully spreading it over the fruit in the first pan, and smoothing the plain batter in the second pan.
Bake for 30-35 minutes until a toothpick comes out clean.
Remove from oven. Let the plain cake cool completely in its pan. Let the pineapple layer rest for exactly 10 minutes, then invert it onto a wire rack to cool.
Once completely cool, place the plain cake on a serving platter. Carefully place the pineapple layer directly on top. Slice and serve!