Preheat your oven to 350°F (175°C) and prepare a 9-inch springform pan by greasing it.

Cream softened unsalted butter and granulated sugar together until light and fluffy.

Add the eggs one at a time, mixing on low speed until just combined.

Mix the all-purpose flour and baking powder in a separate bowl, then gradually add it to the wet ingredients with milk.

Slice the plums in half and arrange them cut-side up over the batter.

Sprinkle ground cinnamon over the plums and optionally dust with sugar.

Bake for 30-35 minutes until edges are golden brown and a toothpick comes out clean.

Cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.