Preheat the oven to 350°F (175°C). Lightly grease a 12-cup muffin tin or use paper liners. The oven should be fully preheated so the caramel and cake set quickly.

In a small bowl, stir the melted butter and brown sugar until smooth. Spoon about 1 teaspoon of the mixture into the bottom of each muffin cup, spreading it evenly.

Place one pineapple ring on top of the caramel in each cup. If using cherries, tuck one in the center of each ring.

In a medium bowl, whisk together the flour, sugar, baking powder, and salt. In a separate bowl, whisk the milk, oil, egg, and vanilla. Pour the wet ingredients into the dry and stir gently until just combined.

Spoon the batter over the pineapple, filling each cup about two-thirds full. Smooth the tops lightly with the back of a spoon.

Bake in the preheated oven for 18 to 22 minutes, or until the cake springs back when lightly pressed.

Allow the tin to cool on a wire rack for 8 to 10 minutes. Run a thin knife around each cake to loosen. Invert quickly but carefully onto a baking sheet or plate.

Serve these warm or at room temperature.