Preheat your oven to 350 degrees. Grease a square baking pan (9 x 9 or thereabouts).
Slice the plums (I used red this time, but dark plums work great too) and set aside in a bowl. I drizzled a little Croatian Sljivovica (our plum brandy) over the fruit for flavor, and you can also do so with brandy or rum.
Mix the flour, baking powder, lemon rind, orange rind, salt, and walnuts together. Set aside.
Cream the butter and sugar until light and fluffy.
Add in the eggs, one at a time, followed by the vanilla extract. Slowly add in the flour mixture until incorporated.
Spoon half of the batter into the pan and smooth out. Add half of the sliced plums (or cherries) evenly. Spoon the rest of the batter over the top and flatten out, using an offset spatula. Place the other half of the sliced fruit on top (I arranged into a fan with 3 slices in each corner; you can get creative in any way you like.)
Bake for 50-55 minutes, making sure that the toothpick test comes out clean in the center of the cake.
Allow to cool in the pan, and move to a wire rack after 20 minutes to cool completely.
Serve with powdered sugar, ice cream, creme fraiche, whipped cream, or nothing at all. Makes for a great coffee break cake or a fine breakfast with fruit included!