Start by bringing a small pot of water to a boil. 

Add the pistachios (without the shell) to the boiling water for 2 minutes. Drain the pistachios, rinse them with cold water and then dump them on a paper towel to soak excess liquid.

Peel the skin off the pistachios. This is tedious and will take some time but this will give you a finer crumb and more green color. Let the pistachios dry completely before grinding them up (if they’re wet they’ll turn into a paste). 

Preheat the oven to 350F (177C) convection (325F/163C conventional) and grease and line four 6 inch cake pan with baking spray and parchment paper. 

Place the fully dried pistachios in the bowl of a food processor and pulse at full speed for 2-3 minutes until they’re as ground as they’re going to get (don’t turn it into a paste). 

Add the cake flour, baking powder and salt to the food processor and let it run for a few more minutes at full speed until the pistachios just about blend into the flour mix. 

Add in the oil and beat for another 1-2 minutes, at full speed, until it's pale and fluffy. 

Add the egg whites, 1-2 at a time, beating on medium speed between each addition for about 1-2 minutes, until each egg is fully incorporated and the batter is fluffy. 

Add the vanilla extract and sour cream and beat just until they're combined.

Add half of the dry ingredients to the batter, turn the mixer up to low speed and stream in the half and half, followed by the rest of the dry ingredients. 

Spread the cake batter in the prepared cake pans and bake for 30 minutes, until it releases itself from the edge of the pan and/or springs back if you gently press on the center of it. 

Flip over onto a tea towel and allow the cake to cool completely before frosting.

Bring a small pot with 1/2 cup of water to a boil and lower the heat to simmer.

Once the sugar is dissolved, remove the bowl of egg whites from the heat. Use the whisk attachment to beat the egg whites with the sugar. Beat for 7-10 minutes on high speed and then lower the speed to low-medium.

Slowly add 2 Tbsp of butter in at a time. Wait for the butter to incorporate before adding the next chunk. Once all the butter is added, turn the mixer up to high speed for a few seconds, until it comes together.

Meanwhile, melt your white chocolate. I do 20 second intervals in the microwave, stirring between each interval until it’s melted. Allow it to cool slightly (not enough to re-harden). 

Use a rubber spatula to scrape the edge of the bowl of buttercream, add the vanilla bean paste (or extract), the melted white chocolate and the salt and beat until it's fully combined. (DON’T stream in the white chocolate while the mixer is running or you’ll cool the melted chocolate as your adding it and you’ll get little hardened bits of white chocolate chunks throughout the buttercream). 

Place the first layer of cake onto a flat surface and frost with a generous layer of buttercream. 

Push the frosting a little bit higher around the edges to create a little edge. Spread the ground pistachios on top of the buttercream, leaving the higher edge exposed so it sticks to the next layer of cake.

Place the next layer of cake on top and repeat until all the layers are assembled.

Refrigerate the cake for 20-30 minutes and then use the rest of the buttercream to lightly frost the outside (double buttercream recipe for fancy decorating).

Top with more ground pistachios and serve slightly chilled or at room temperature.