Preheat the oven to 375°F (190°C).

Cut the plums in half and remove the pits.

In a stove-safe pan, melt the butter over medium heat and add the sugar. Stir until the mixture turns golden brown.

Arrange the plum halves in the pan, cut side down. Sprinkle cinnamon over the plums.

Roll out the puff pastry and place it over the plums, tucking the edges into the pan.

Bake in the preheated oven for 40 minutes or until the pastry is golden and crisp.

Let the tart cool for 10 minutes, then invert it onto a serving plate.