Preheat the oven to 350 degrees F. Grease the baking pan/dish.
Cream the butter in a large bowl with sugar, lemon zest, and vanilla until light and fluffy. Sift the flour, baking powder and salt together, and beat into the creamed butter.
In a separate bowl, beat the eggs until they start to foam. Do not over beat, or the cake will be tough.
Add the egg and ground walnuts to the flour and butter mixture. Mix well.
Scrape the batter into the prepared pan/dish. Arrange the Prunes on top in form of rings. Sprinkle a little sugar.
Bake for 1 hour, or until the cake is golden brown on top and a toothpick inserted into the cake comes out clean.
Let it cool for 10 minutes before removing the sides of the spring from pan.
Garnish with a dusting of confectioners’ sugar and serve.