Preheat oven to 350°F (177°C). Grease and lightly flour three 9-inch cake pans.Pulse the pistachios in a food processor until ground up into very fine crumbs. See photo above for a visual. You'll have about 1 and 1/2 cups of crumbs. (If you have more than that, set aside for garnish.)

Pour 1 and 1/2 cups of pistachio crumbs in a large bowl. Whisk in the cake flour, baking powder, baking soda, and salt. Set aside.

Using a handheld or stand mixer fitted with a paddle or whisk attachment, beat the butter and sugar together on high speed until smooth and creamy, about 3 minutes. Scrape down the sides and up the bottom of the bowl with a rubber spatula as needed. Beat in the egg whites on high speed until combined, about 2 minutes. Beat in the sour cream, vanilla extract, and almond extract. Scrape down the sides and up the bottom of the bowl as needed. With the mixer on low speed, add the dry ingredients until just incorporated. With the mixer still running on low, pour in the milk (and the green food coloring, if using) and mix just until combined. You may need to whisk it all by hand to make sure there are no lumps at the bottom of the bowl. The batter will be slightly thick.

Pour batter evenly into cake pans. Bake for around 21-23 minutes or until the cakes are baked through. To test for doneness, insert a toothpick into the center of the cake. If it comes out clean, it's done. Allow cakes to cool completely in the pans set on a wire rack. The cakes must be completely cool before frosting and assembling.

Using a large serrated knife, slice a thin layer off the tops of the cakes to create a flat surface. Discard (or crumble over ice cream!). Place 1 cake layer on your cake stand, cake turntable, or serving plate. Evenly cover the top with frosting. Top with 2nd layer and evenly cover the top with more frosting. Top with the third cake layer. Spread the frosting all over the top and sides. (I used a bench scraper to smooth out the sides.) Decorate with garnishes, if desired. Refrigerate for at least 30-45 minutes before slicing. This helps the cake keep its shape when cutting.

Cover leftover cake tightly and store in the refrigerator for 5 days.