Preheat the oven at 350° F.
In a big bowl, sift flour, grounded pistachios, baking powder, baking soda and salt together. Mix using a spoon and set aside.
In another bowl, with a hand mixer, cream together the butter and 1 cup of sugar for 2 minutes until you get a fluffy and lighter in color mixture.
Beat in the vegetable oil and then the egg yolks, one at a time, beating after each addition.
Add in the vanilla and the pistachio paste and beat for 3 more minutes until the mixture is really fluffy.
Incorporate the previously sifted dry ingredients in three additions, alternating with the buttermilk, starting and ending with the dry igredients (once you add in the dry ingredients, it is important not to beat to much, just until the mixture is homogenized).
Making sure the beaters of the mixer are clean and completely dry, in another clean bowl, beat the egg whites with the rest of the sugar (3/4 of a cup) and the cream of tartar for 4 minutes aprox. until the egg whites turn fluffy and firm.
Incorporate the egg whites to the main mixture in three additions using a rubber spatula, mixing form the outside in.
Divide the batter by pouring it in two cake pans (9 inch -23cm- in diameter), previously greased and floured, and bake for 40 minutes or until a wooden stick comes out clean when inserted.
Let the cakes cool down for 10 min. inside the pans and then turn them out and place them on a cooling rack for them to cool down completely.
In a big bowl, cream the cream cheese, butter and confectioners sugar together using a hand mixer until you get a fluffy mixture.
Beat in the salt, vanilla and pistachio paste until it all incorporates.
In a small microwave-safe bowl, mix the marshmallows with the vegetable oil so that they all end up coated (you can add some more oil if necessary). Microwave them for 10 seconds, then mix using a spoon to make the marshmallows start melting; repeat this once or twice more until the marshmallows form a sort of sticky cream or fluff. Immediatly add this fluff to the cream cheese mixture and beat until well incorporated.
Incorporate the condensed milk and mix for 1 more minute until you get a soft creamy mixture.
Lastly, add the whipping cream and beat for 20 seconds, just until incorporated, otherwise the mixture will curdle.
Place this in a pipping bag without a tip (it won´t be necessary) and fridge it for 1 hour or until you need it.
In a small pot, place the milk along with half of the sugar and the vanilla seeds and heat it to low heat, whisking occasionally using a hand whisk.
In a small bowl, mix the rest of the sugar with the egg yolks and the corn starch. Add in a little of the milk mixture and mix for the temperatures to even out.
Pour the egg yolk mixture into the pot with the milk and cook to medium heat for 3 minutes aprox., stiring nonstop, until it thickens.
Turn off the heat, pour this cream in a clean bowl, cover it with plastic wrap (which must be touching the cream surface, otherwise a custard will form) and refrigerate for 1 hour.
Meanwhile in a big bowl, using a hand mixer, cream together the cream cheese, butter, confectioners sugar and salt. Beat for 3 minutes or until the mixture turns fluffy and lighter in color.
Add the refrigerated cream to this (which must already be cold) and mix until it´s all well incorporated and the frosting turns out soft and creamy. Cover the bowl with plastic wrap and refrigerate until necessary.
Take the filling and the frosting out of the fridge.
With a really sharp knife, cut each cake in half to get 4 thinner cake layers.
Place one of the layers on a big plate or a cake stand (which is where your cake will be presented). With the pipping bag put enough filling and spread all over the surface of this piece of cake, and then evenly place 1/2 C natural pistachios, chopped. Repeat his step twice more finishing with layering the fourth and final piece of cake on the top.
Cover the cake with a lid and refrigerate for 30 minutes, this will make all of the layers sit and make it easier for us to frost the cake later on.
Frost the cake the way you want to and decorate with some more natural chopped pistachios.
*The pistachio paste is obtained by smashing pistachios which, given that they have a high vegetable fat concentration, make a pure paste that can be used to flavor a lot of recipes. It can be found at baking supply shops.
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