- Preheat the oven to 350°F (180°C) and grease an 8-inch (20 cm) cake pan. Place the pan on a parchment lined baking tray.
- Bring the butter and brown sugar to a simmer in a small saucepot while stirring. Once simmering, add the orange juice and return the pot to a simmer. Pour the syrup into the prepared pan and arrange the plum halves overtop of the syrup, with the plums skin-side down.
- Using a food processor, pulse the hazelnuts and sugar until the hazelnuts are finely ground. Add the oil, ricotta, eggs, vanilla and orange zest and pulse to blend. Add the flour, baking powder and salt and pulse just until combined. Gently scrape the batter overtop the plums. Bake the cake for 50-55 minutes, until a tester inserted in the centre of the cake comes out clean. Allow the cake to cool for about 40 minutes, then place a serving plate over the baking pan and invert the pan to tip the cake out. Serve the cake warm or at room temperature.