Preheat the oven to 350°F. Butter an 8x8-inch baking pan, line the bottom with parchment paper and butter the paper.

In a food processor, combine the pistachios and lemon zest, and pulse until finely ground. Transfer to a bowl and mix with flour, baking powder, and baking soda. Whisk to combine, and set aside.

In another small bowl, whisk together the sour cream, vanilla, and milk, then set aside.

In the bowl of a stand mixer with a paddle attachment, cream the butter, sugar, and salt at medium speed until light and fluffy, about 3 minutes.

Add the eggs one at a time, ensuring each is fully incorporated. Scrape the sides and bottom of the bowl as necessary.

With the mixer on low, alternate between adding the dry ingredients and the wet ingredients, starting and finishing with the dry ingredients.

Pour the batter into the prepared pan and smooth the top. Bake for 35-40 minutes or until a toothpick inserted in the center comes out clean.

Allow to cool in the pan for about 10 minutes. Carefully turn the cake out onto a work surface, remove the parchment, then invert onto a wire rack to cool completely.

Spread the frosting on top, sprinkle with chopped pistachios, and serve at room temperature.

In a medium bowl, combine all frosting ingredients. Mix with a hand mixer until smooth, being careful not to overmix as the frosting may curdle.