Each bite of thismoist coffee cake holds a hint of cinnamon, sweet Italian prune plums, and pinons.
Make streusel: Put all streusel ingredients in a bowl. Using a pastry cutter or two knives, blend until mixture resembles coarse meal. Refrigerate.
Make crumb topping: Put all topping ingredients in a bowl. Using a pastry cutter or two knives, blend until mixture resembles coarse meal. Refrigerate.
Make cake: Preheat oven to 375 degrees. Butter and flour a 10-inch tube pan. Sift flour, baking powder, baking soda, and salt together into a medium bowl; set aside.
Put butter and granulated sugar in the bowl of an electric mixer fitted with the paddle attachment. Cream on medium-high speed until pale and fluffy, 2 to 3 minutes. Add beaten eggs in two batches, mixing well. Mix in one-third of flour mixture. Mix in half of sour cream. Repeat with remaining flour and sour cream. Stir in plums by hand.
Pour half of the batter into prepared pan; sprinkle with streusel. Top with remaining batter. Pour vanilla on top. Bake 20 minutes. Sprinkle with crumb topping; bake until golden brown, 25 to 30 minutes more. Let cool on a wire rack. Unmold. Dust with confectioners' sugar.
Pinon is Spanish for "pine nut."