Cut 5 pieces of parchment paper, each with dimension of approximately 35×35 cm (14″x14″). Use a soft graphite pencil to trace or inscribe 26 cm (10″) circle on each paper, set aside.
Place full size baking sheet upside-down on the oven rack that is placed in the middle, Preheat the oven to 160°C(320°F).
Separate egg whites to a large, grease free mixing bowl. Using a hand held mixer or a stand mixer beat the egg whites at a medium speed. While mixing gradually add the sugar and carefully monitor mixing to achieve soft peaks.
Using a large silicon spatula and through few consecutive batches carefully incorporate ground walnuts into the merengue. Do not overmix it and deflate the meringue!
Take the parchment paper, turn it upside down (escribed circle on the bottom), put it on the scale and weigh 1/5 of the walnut meringue mixture. Using an offset palette knife spread the meringue layer evenly to cover the marked circle shape area.
Using a pizza peal transfer the parchment paper to prewarmed oven. Bake for about 15 minutes or until walnut meringue layer start browning around the edges.
Put it on a cooling rack for 5 minutes. Turn it upside down and carefully peel off the parchment paper. Transfer baked layer to a new peace of clean parchment paper and let cool completely.
Repeat the process with another 4 walnut meringue layers.
Ganache Filling
Over double boiler melt the chocolate, let cool off slightly, about 5 minutes or until the temperature drops under 40 °C(104ºF).
In a large mixing bowl at low speed start whipping the cream, add sugar in batches.
As the cream start increasing in volume due to incorporated air (not fully whipped yet!) slowly pour in melted chocolate in a thin stream. Mix until fully incorporated and smooth ganache texture is achieved.
Note:
If your mixer beaters and mixing bowl are too cold (it happened to me) after the chocolate is mixed in the ganache might be little too stiff. If that happens do not despair, just place the metal mixing bowl on a stove and warm it up just slightly at low heat. Use silicone spatula to mix constantly. This will make filling soft and easily spreadable. You can repeat this process again if necessary during the assembling process.
Assembly
Place a small dollop of ganache in the middle of the serving dish, place the first meringue layer over it. This will assure that the layer doesn’t slide around during assembly.
Weigh the ganache cream. Using an offset pastry knife spread evenly 1/6 of the weight onto the first meringue layer.
Repeat the same with another 4 meringue layers.
Use the last 1/6 of ganache cream to smoothen the sides of the cake. Refrigerate the cake for 1 hour.
Glaze
Over double boiler melt the chocolate, mix in the oil. Let cool off slightly, about 5 minutes or until the temperature drops under 40 °C(104ºF).
Slowly pour the chocolate on top of the cake trying to cover the full surface. Do not spread the glaze with knife or any other tool!
Just tilt the cake in all directions until the top is fully coated and most of the cake side. Use an offset pastry knife to quickly even the glaze on the cake sides. Add chopped pistachios around the cake base while glaze is still soft.
Refrigerate for 1 hour.
Decorating
Over double boiler melt the chocolate, let cool off slightly, about 5 minutes or until the temperature drops under 40 °C(104ºF).
In a medium mixing bowl at low speed start whipping the cream.
As the cream start increasing in volume due to incorporated air (not fully whipped yet!) slowly pour in melted chocolate in a thin stream. Mix until fully incorporated and smooth ganache texture is achieved.
Use a pastry bag fitted with a large open star piping tip to pipe 16 shells around the cake perimeter. Sprinkle chopped pistachios on the inside end of each shell.
Cover and refrigerate the cake, best overnight before serving.