Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 9x13 inch pan.
In a medium bowl, stir together the flour, baking powder, baking soda, and salt.
In a large bowl, cream together the butter and sugar until light and fluffy. Beat in the eggs one at a time, then stir in the vanilla. Gradually beat in the flour mixture alternately with the buttermilk.
Fold in the prunes and nuts. Spread the batter into the prepared pan.
Bake in the preheated oven for 40 to 45 minutes, or until a toothpick inserted into the center of the cake comes out clean.
Cool in the pan for 10 minutes, then turn out onto a wire rack and cool completely.
Interesting Facts
Prunes are a great source of fiber, vitamins, minerals, and antioxidants.
Adding walnuts to this cake adds a crunchy and nutty flavor.
This cake is best served warm with a scoop of vanilla ice cream.