Preheat oven to 325 degrees. Spray an 8 × 8 baking pan with a flour based baking spray and line bottom and sides of your dish with parchment paper for the best results.
Heat 1/2 cup prunes with 1/3 cup Armagnac in a small saucepan over medium until warm, 2 to 3 min. Remove from heat and let soak while you continue.
Sift flour with cocoa powder, baking powder, baking soda and salt into a large mixing bowl and set aside.
Combine 3/4 cup prunes with yogurt and vegetable oil in a food processor until it turns into a light and shiny paste. Add paste to flour mixture and add in egg, sugars and molasses.
Add in chopped chocolate and buttermilk. Your mixture will be rather dry. Scrape mixture into prepared baking pan. Smooth top. Cut soaked prunes in half and place on the top of mixture, pressing them in a bit lower than the surface. Reserve remaining liquid for the syrup.
Bake until a skewer inserted into the centre of cake comes out clean, about 45 to 50 min. Set on a cooling rack.
To make a syrup combine water with 1/3 cup sugar in a small saucepan over medium. Stir until sugar has dissolved, 2 to 3 min. Cool for 10 min, then stir in 2 tbsp Armagnac and remaining liquid from prunes.
Poke several holes in the warm cake with a skewer. Pour syrup evenly over cake. Let cool completely on a rack, 1 hour.
Garnish with extra chocolate shavings and powdered sugar if desired. We liked eating it a bit warm at our house. You can always pop a slice into the microwave for a few seconds too.