Preheat the oven to 170°C (340°F) and line a baking tray with parchment paper.
Wash and cut the plums into quarters, selecting firmer ones for better texture.
In a large bowl, whisk the eggs with sugar until the mixture becomes pale and creamy.
Gradually add the oil while whisking continuously.
Mix in the sour cream until fully incorporated.
Stir in the vanilla essence, lemon zest, and a pinch of salt.
Gradually sift in the flour, folding gently after each spoonful to keep the batter airy.
Before the last tablespoon of flour, mix in the baking powder.
Pour the batter into the prepared baking tray and spread it evenly.
Arrange the plum slices on top, placing them close together.
Bake for 35-40 minutes, or until the cake is golden brown and a toothpick inserted in the center comes out clean.
Allow the cake to cool completely before cutting into squares. Dust with powdered sugar before serving.