Preheat the oven to 356º F (180° C). Line a square cake pan of about 9 inches (23 cm). In a medium bowl, sift the flour with the baking powder, baking soda and salt.
To grind the pistachios, place them in a food processor or blender and grind for a few seconds until they are completely ground.
In a large bowl, add the butter with the sugar and cream with an electric mixer or hand whisk for 2-3 minutes until light and fluffy.
Mix in the eggs one at a time, beating for 20 seconds after each addition. Add the vanilla extract with the sour cream and beat for 1 more minute until very smooth.
Add the flour mixture in 2 separate additions, alternating with the buttermilk and beating after each addition. Beat for 10 seconds until smooth. Add the ground pistachios and mix with a rubber spatula just until incorporated.
Pour the mixture into the cake pan and bake at 356º F (180° C) for approximately 40 minutes until a wooden toothpick comes out clean when inserted.
Once ready, take the cake out of the oven and let it cool down in the pan while preparing the rest of the recipe.
For the frosting:
In a large bowl, place the cream cheese with the butter, powdered sugar and salt. Beat with an electric mixer or a hand whisk for 4 minutes until the mixture lightens and fluffs a little.
Add the whipping cream and beat for 1 more minute just until the mixture becomes a little thicker and creamier. It is important not to over whip. Place the frosting in the refrigerator until needed.
For the pistachio cream:
Add the pistachios with the condensed milk to the same food processor or blender that had been used to grind the pistachios for the cake, and blend for about 5 minutes until a very smooth, slightly runny pistachio paste forms.
Pour this mixture into a medium bowl, add the whipping cream and beat with the same electric mixer or hand whisk that was used for the frosting for about 3-4 minutes until the cream is very smooth and forms stiff peaks.
Place the cream in a pastry bag or ziploc bag with a round small pastry tip and reserve in the refrigerator until needed.
To assemble the cake:
When the cake is completely cool, carefully turn it out and place it on a large serving plate. Poke many holes in the top of the cake using a wooden toothpick or the handle of a wooden spoon.
Make a small cut in the piping bag with the pistachio cream to be able to fill the cake, introducing the pastry tip with cream into the holes that had been made.
Carefully add the frosting on the top of the cake, using a rubber spatula to spread it over the entire surface and form a thick frosting layer.
To decorate, add some more of the leftover pistachio cream on top and swirl it to make a similar to marble pattern using a spoon. Sprinkle the chopped pistachios on top and its ready!