Once the mixture is light and fluffy, lower the mixer to its lowest speed. Add 4 large eggs, one at a time, only adding the next egg after the previous egg has been fully incorporated into the batter. Once all the eggs have been incorporated, add 1 tablespoon vanilla and continue mixing until just combined.
Continue to keep the mixer at its lowest speed and add the dry ingredients (from the 2nd step) in three additions, alternating with 1 cup sour cream in two additions, beginning and ending with the dry ingredients. After each addition, beat until just combined — DO NOT OVERMIX at this stage, or you'll end up with a dense, tough cake that will make me sad.
Once the last of the dry ingredients have just been incorporated into the batter, turn off your mixer. Use a rubber spatula to fold in 1/2 cup pistachio paste, making sure to not overmix the batter. Once the pistachio paste has been fully incorporated and the batter has an even, slightly grainy texture, stir in 1/2 teaspoon green food coloring until just combined. There will be a lot of batter at this point, so don't worry if it takes some time to fully incorporate the food coloring.
Once the batter is an even green color throughout, divide the batter evenly betwen the two pans and spread the batter out evenly with a rubber spatula. Gently tap each filled pan on the counter 4 or 5 times to remove any air bubbles in the batter.
Bake the cakes in the preheated oven for 35 to 45 minutes, or until the top is nicely browned and a skewer inserted into the center of the cake comes out clean. When poked with your finger, the top of the cakes should gently spring back. Transfer to a wire rack and allow to cool completely before frosting and assembling.
For the Dark Chocolate Ganache Frosting and Pistachio Topping
In a small saucepan over medium heat, bring 3/4 cup plus 1 tablespoon heavy cream to a gentle simmer, whisking occasionally. Pour the hot cream into the heatproof bowl containing the chocolate and confectioner's sugar mixture (from the 1st step). Stir the mixture with a heatproof rubber spatula until the sugar is dissolved.
Once the sugar has dissolved, make a double boiler by nestling the heatproof bowl over a medium saucepan filled with water — be sure that the water in the saucepan doesn't touch the bottom of your heatproof bowl. Gently cook, stirring with a heatproof rubber spatula, until all the chocolate has melted and the mixture is smooth. Remove bowl from the heat.
In a separate, smaller heatproof bowl, whisk together 2 large egg yolks. Temper the yolks by pouring about 1/2 cup of some of the melted chocolate mixture (from the 3rd step) into the yolks, stirring constantly. Pour the tempered mixture back into the larger, heatproof bowl containing the rest of the original mixture and whisk to combine. Add 3 tablespoons unsalted butter and continue whisking until smooth.
Allow the ganache to cool for about 10 - 15 minutes, before using immediately. Spread around 1 1/2 cups of frosting at the top of each layer and sprinkle each layer with 1 cup finely-ground pistachio nuts. Gently nestle one layer on top of the other and voila! A two-layer cake. Enjoy!
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