Preheat oven to 325 degrees F. Spray an 18-cup tube pan with nonstick baking spray. Add pecans to a baking sheet and toast in the oven for 6-8 minutes, until slightly golden brown. Cut the prunes and pecans into small pieces. Set aside.
Begin by making the streusel - combine all purpose flour, sugar, cinnamon, and a pinch of kosher salt in a large bowl. Mix together, then add the room temperature butter and shortening. Use your fingers or a fork to pinch the butter into the flour mixture until crumbles begin to form and you can no longer tell the butter from the dry ingredients. Place the bowl into the refrigerator.
Time to make the cake -In a separate bowl, whisk together flour, chopped pecans, cinnamon, allspice, nutmeg, baking powder, and kosher salt. In the bowl of the stand mixer fitted with the paddle attachment, add butter, shortening, sugar, and brown sugar. Cream together for 5 minutes, until light and fluffy.
Reduce the mixer speed to low and begin adding eggs, one at a time, ensuring each egg is fully incorporated prior to adding the next. Scrape down the sides of the bowl between each addition of the eggs.
Add half of the dry ingredients to the wet ingredients and mix for about 20 seconds, until just combined, then add in the milk, sour cream, and vanilla extract. Continue to mix until just combined, about 10-15 seconds. Scrape down the sides of the bowl again, then add the remainder of the flour mixture. Mix until the batter is smooth, about 10 seconds. Scrape down the sides and the bottom of the bowl one last time to ensure that all of the batter is well mixed, then fold in the chopped prunes.
Pour batter into the prepared pan. Remove the streusel topping from the refrigerator and sprinkle on top of the batter, ensuring it is fully covered. Bake in the preheated oven for 1 hour and 20-30 minutes, until a toothpick comes out with just a few crumbs.
Allow it to cool in the pan for 30 minutes, then remove onto a wire rack to cool completely.
Once the cake is cooled, add dulce de leche to a large mixing bowl and add whole milk. Whisk together until combined, then drizzle on top of the cooled cake.
Notes
** For the dulce de leche drizzle, you can use store bought or make your own at home. If making your own, you'll need a can of 14 oz sweetened condensed milk. Simply can for 3 hours (adding more water as it begin to evaporate, ensuring the can is submerged). Allow it to cool completely, then open and proceed.
Use room-temperature ingredients: Butter, eggs, and sour cream should all be at room temp—this helps them blend evenly for a smooth batter and consistent bake.
Test for doneness at the center: Prune cake is dense and moist, so a toothpick should come out clean from the center of the cake or with just a few moist crumbs—don’t rely on timing alone.
Let it rest before slicing: Cool completely to allow the flavors to meld and the crumb to set—it's even better the next day!
Serve the warm cake with a scoop of ice cream for the ultimate dessert!