Preheat the oven to 350°F (175°C) and generously grease a 12-cup muffin tin, then coat cups with nonstick spray.

Stir melted butter and brown sugar in a small saucepan over medium heat until bubbly, about 2 minutes. Divide mixture among muffin cups, top each with a pineapple ring and a cherry, if using.

Whisk flour, baking powder, and salt in a medium bowl. In a large bowl, whisk sugar, egg, vanilla, and oil until smooth. Alternately stir in dry ingredients and pineapple juice, mixing just until combined. Spoon batter over toppings, filling cups two-thirds full.

Bake 20 to 22 minutes, until tops are golden and a toothpick comes out clean. Cool in the pan exactly 2 minutes, then run a knife around each cake.

Invert the tin onto a baking sheet or cutting board, lift the tin away, and serve warm.