For the Cake:

1. Preheat the oven to 350F. Spray two 9-inch cake pans with nonstick spray and set aside.
2. Pulse the pistachios in a food processor until very fine crumbs form. Place the crushed pistachios in a bowl and whisk in the flours, baking powder, and salt. Mix until well combined.
3. Place the chocolate chips/shavings in a small mesh strainer and place it over the bowl with the flours. Add a couple tablespoons of the flour mixture to the chocolate chips and shake the strainer gently so that the flour mixture coats the chocolate and falls back into the bowl. Set the chocolate aside.
4. In the bowl of an electric mixer, cream together the butter and sugar until very light and fluffy, about 3 minutes. Add the eggs, one at a time, beating after each addition. Add in the vanilla and beat to combine.
5. Add in half of the pistachio-flour mixture, beating to incorporate. Stream in the buttermilk and beat to combine. Add in the remaining dry ingredients and beat until combined, scraping down the sides of the bowl as needed.
6. Divide the batter evenly between the two prepared cake pans. Place in the preheated oven and bake for 30-35 minutes or until the cakes are golden brown and set in the middle. Cool the cakes in the pans for 20 minutes and then carefully invert them onto cooling racks. Cool them completely and then refrigerate them for at least an hour before frosting and assembling.

For the Frosting:

1. While the cakes are chilling, beat the butter, cream cheese, and sugar together in the bowl of an electric mixer until creamy, light, and fluffy. Add in the vanilla bean paste and 1 tablespoon of the milk, beating well to incorporate. Add more milk, as needed, to thin out the frosting to a spreadable consistency. Beat in the green food coloring, if using. Transfer half the frosting to a piping bag fitted with a large round tip.

To Assemble:

1. Place the first chilled cake layer on a cake stand or serving platter. Pipe a ring of the frosting around the top edge of the cake, creating a barrier for the frosting filling. Fill the middle of the cake with a few spoonfuls of frosting and spread everything out so it is even. Top with another cake layer and repeat the frosting process.
2. Decorate as desired with meringues, glitter sugar, a dusting of powdered sugar, and sprinkles.
3. Serve!