Preheat the oven to 350 degrees. Line a 13 by 9-inch baking sheet with parchment paper and grease the paper with nonstick spray.
In a food processor, grind the nuts to a fine powder. You can also chop them very finely in a pinch.
In the bowl of an electric mixer fitted with the paddle attachment, cream the butter and sugar until light and fluffy, 4-5 minutes.
Add the eggs and yolk one at a time, mixing until incorporated and scraping the bowl well after each addition. Beat in the vanilla extract.
In a medium bowl, whisk the flour, baking powder, and salt to combine. Add 1/3 of the mixture to the butter mixture, mixing until incorporated. Add 1/3 of the milk and mix to combine. Repeat two more times until all of the wet and dry ingredients are incorporated and the batter is smooth. Fold in the ground pistachios.
Pour the batter into the prepared pan and use a spatula to smooth it as evenly as possible. Bake until the cake springs back lightly when touched, 15-20 minutes. Cool completely.
When the cake is cooled, cut out the layers. I used a paring knife and a small 5-inch cocktail plate as a guide, tracing around the plate to make the layers. I got 4 layers out of the baking sheet. You can use a slightly larger or smaller guide, just know it might affect the number of layers. Set aside.
To make the whipped cream, sprinkle the gelatin over the cold water. Let sit for 5 minutes. In a small pot, melt the gelatin mixture until dissolved. Cool slightly.
In the bowl of an electric mixer fitted with the whip attachment, whip the cream and vanilla on medium speed. Gradually add the sugar. When the mixture starts to look thick, gradually add the gelatin mixture. Continue to whip to medium peaks.
Layer the cake with frosting in between each layer and frost all around. Garnish with chopped pistachios and cherries, if desired.
I always carry three kinds of hot sauce in my purse. I have a soft spot for making people their favorite dessert, especially if it's pie. My first cookbook, The Fearless Baker, is out on October 24, 2017.