Preheat the oven to 180°C (350°F). Grease and line a 20–23 cm (8–9 in) round cake pan.

Wash, halve and pit the plums; set aside cut side up so excess juice drains slightly.

In a bowl whisk together flour, baking powder, salt and optional cinnamon; set aside.

In a large bowl beat softened butter and granulated sugar until pale and fluffy, about 3–4 minutes.

Add eggs one at a time, beating after each addition until incorporated; stir in vanilla and lemon zest.

Fold in one-third of the dry mixture, then half the milk, repeating until batter is smooth and evenly combined.

Spoon batter into the prepared pan, smoothing the top so it reaches the edges evenly.

Arrange plum halves on top, cut side up, pressing them gently so they sit slightly into the batter; sprinkle demerara or brown sugar over the plums.

Bake near center of oven for 40–50 minutes, or until a skewer inserted near the center (avoiding plums) comes out clean and the cake is golden.

If the plums release a lot of juice and the top browns too quickly, tent loosely with foil during the last 10 minutes.

Remove cake from oven and cool in pan for 10–15 minutes, then transfer to a wire rack to cool completely or serve slightly warm. Optionally brush the plums with a spoonful of warmed jam for shine.