Preheat your oven to 350°F (175°C). Grease a muffin tin or line it with cupcake liners.
In each muffin cup, place one pineapple slice. If needed, cut the slices to fit the size of the cups. Place a maraschino cherry in the center of each pineapple slice.
In a small bowl, combine the melted butter and brown sugar. Mix well until the sugar has dissolved. Spoon the mixture evenly over the pineapple slices and cherries.
In a separate bowl, whisk together the flour, baking powder, and salt.
In a large mixing bowl, beat together the granulated sugar, vegetable oil, eggs, and vanilla extract until well combined.
Gradually add the dry ingredients to the wet ingredients, alternating with the pineapple juice. Begin and end with the dry ingredients, mixing until just combined. Do not overmix.
Spoon the batter over the pineapple slices in the muffin cups, filling each cup about 3/4 full.
Place the muffin tin in the preheated oven and bake for 20-25 minutes, or until a toothpick inserted into the center of a cake comes out clean.
Remove the muffin tin from the oven and let it cool for a few minutes. Then, carefully invert the tin onto a wire rack or serving platter to release the mini pineapple upside-down cakes. Allow them to cool completely before serving.
Serve the mini pineapple upside-down cakes as they are, or you can add a dollop of whipped cream or a scoop of vanilla ice cream on top for extra deliciousness