Preheat the oven to 350°. Grease a heavy nonstick 10-inch Bundt pan. In a large saucepan, melt the butter over moderate heat, adding the orange zest halfway through. Remove from the heat.
Using a wooden spoon, stir in the sugar, vanilla, salt, sour cream and eggs until smooth. Add the baking soda and baking powder in pinches, breaking up any lumps with your fingers; mix in. Vigorously mix in the flour. Spoon the prune puree over the batter; sprinkle the cinnamon, cocoa and walnuts evenly on top. Swirl in these ingredients.
Pour the batter into the prepared pan and bake for 50 to 55 minutes, or until a tester inserted in the center comes out with a crumb or two clinging to it. Let the cake cool in its pan on a rack for 10 minutes; then invert it onto the rack and let cool completely.
The coffee cake can be wrapped and refrigerated for up to 5 days.
Prune puree, also called prune pastry filling or lekvar, is available at most supermarkets.