Preheat the oven to 180°C (350°F).
Heat an ovenproof skillet over medium heat. Add the butter and bay leaves and allow to melt and bubble. Add the sugar and maple syrup and stir to dissolve. Cook for a minute or so until bubbling, then add the cinnamon and vanilla. Cook for a few more seconds, then turn off the heat and remove the bay leaves.
If using a cake tin, carefully pour the caramel into the base of the tin now.
Arrange the plum halves cut side down in the caramel, fitting them snugly in a single layer. Drape the puff pastry over the top, tucking the edges in around the plums.
Bake for 25 to 30 minutes until the pastry is deep golden brown.
Remove from the oven and allow to rest for 2 minutes. Place a serving plate larger than the pan firmly on top, then flip in one confident motion. Leave the pan on the plate for a minute before lifting it off so the caramel drips over the fruit.
Serve warm or at room temperature with vanilla ice cream or whipped cream.