Preheat the oven to 350°. In a medium saucepan, combine the granulated sugar, white wine, vanilla bean, cinnamon stick, orange zest and 3 cups of water and bring to a boil. Simmer over moderate heat for 10 minutes. Add the prunes and simmer over moderately low heat until plumped and tender but not falling apart, about 25 minutes. Using a slotted spoon, transfer the prunes to a plate to cool.
Coat eight 1-cup ramekins with vegetable oil spray and set them on a sturdy rimmed baking sheet.
In a small saucepan, combine the brown sugar and butter with 1/4 cup of water and bring to a boil, stirring to dissolve the sugar. Let cool slightly. Divide the caramel and poached prunes among the ramekins.
In a medium bowl, whisk the orange juice with the buttermilk, eggs, oil, granulated sugar and 1/4 cup of the yogurt. In a large bowl, whisk the flour with the baking powder, baking soda and salt. Whisk the egg mixture into the dry ingredients until smooth. Divide the batter among the ramekins, filling them a little more than half full. Spray a sheet of parchment paper with vegetable oil spray and invert it over the ramekins. Set another baking sheet on the parchment.
Bake the cakes for about 1 hour and 10 minutes, until a tester inserted in the centers comes out clean. Let the cakes cool for 10 minutes, then run a thin knife around the edges and invert them onto plates, allowing any caramel in the ramekins to drip over the cakes. Spoon a dollop of the remaining 1/2 cup of yogurt on top of each cake and serve.