Heat oven to 350°F. Grease a 9-inch round cake pan and set aside.
Melt the butter in a small pan, then stir in brown sugar until glossy. Spread the mixture evenly in the cake pan.
Arrange pineapple rings over the sugar layer and tuck cherries into the centers and gaps.
Whisk flour, granulated sugar, baking powder, and salt in a large bowl.
In another bowl, whisk milk, reserved pineapple juice, eggs, vanilla, and oil until smooth.
Pour wet ingredients into dry and stir just until combined. Do not overmix.
Scrape batter over the fruit and smooth the top. Tap the pan once to settle.
Bake 35–40 minutes, until the top is deep golden and a toothpick comes out clean.
Cool 10 minutes on a rack. Loosen edges, invert onto a plate, and let set 5 minutes before lifting the pan.
Slice and serve warm. Add whipped cream or ice cream if you like.