Place the prunes, water and dark rum in a small pan.  Bring it to a boil and cook until most of the water has evaporated. 

Remove it from the heat and mash the prunes with the back of a spoon until smooth.  Don’t worry if a few small pieces are left.  

And the remaining ingredients, stirring until the butter melts and all is combined.  Set aside to cool completely.

Line an 8” round cake pan with parchment paper.  Spray the paper and entire cake pan with a non-stick baking release.  Set aside.   

Using the round pan, draw an 8” circle in the middle of a piece of parchment paper.  Turn it over and set aside. Preheat the oven to 350F.  

Cut the butter into small pieces and keep refrigerated until using.   

Reserve one egg yolk for the finish.  Place it in a small bowl and cover it directly with plastic wrap.  

Combine the flour and sugar in the bowl of a food processor.   Pulse to combine.  

Add the cold butter and process until it is indistinguishable.   

Add five egg yolks and process until the dough comes together.   Scrape down. 

Remove the dough from the processor.  

Clean the blade by pulsing several times.  

Bring the dough together in one piece.  It will be soft at this point.  Do not add flour to the work surface.  Just use a gentle motion to bring it together.  It will weigh about 770 grams or about 27 ounces.  Divide it in half or about 385 grams each or 13 ½ ounces each. Shape into discs. 

If the dough is too soft to work, chill it for about 2 hours or so until it firms up.  Do not let it get too hard or it will be difficult to press in and roll out. 

Place one disc in the middle of the pan.  Cover it with plastic wrap and press it out evenly to cover the bottom of the pan. 

Spread the filling to within about ¾ inch from the sides.  

Place the second disc of pastry in the middle of the circle on the parchment paper.  Place another piece of parchment or waxed paper over the pastry and roll out to fit within the 8” circle.   

Flip the paper over so the bottom is on top.  If the pastry does not exactly fit, adjust it now. 

Chill just until firm but not hard, about 10 to 15 minutes. 

In the meantime, stir a teaspoon of water into the reserved egg yolk.

Dampen the edges of the bottom piece of pastry with water and place the top over the filling.  Press the top and bottom edges together lightly.  

Apply the egg wash to the top of the cake.  Using a fork, draw a cross hatch on the top layer. 

Bake for 50 to 55 minutes until the top is a medium brown.  Cover with foil if it browns too quickly.  

Cool to lukewarm, go around the edges of the pan with a small metal spatula and turn the gateau out.  Cool completely right side up.