Preheat your oven to 350°F (175°C) and lightly butter an 8x8-inch baking pan. Line the bottom with parchment paper.

In a food processor, pulse the salted pistachios and lemon zest until finely ground. Combine with cake flour, baking powder, and baking soda in a bowl.

In another bowl, whisk together sour cream, vanilla extract, and milk until smooth.

In a stand mixer, beat softened butter, granulated sugar, and a pinch of salt on medium speed for about 3 minutes until light and fluffy.

Add eggs one at a time, mixing well after each addition for a stable batter.

Gradually add the dry and wet mixtures to the creamed butter and sugar, alternating between the two.

Pour the batter into the prepared pan and bake for 35-40 minutes or until a toothpick comes out clean. Cool in the pan for 10 minutes before inverting onto a wire rack.

For the frosting, beat mascarpone cheese, powdered sugar, and vanilla extract until smooth. Once the cake is cooled, spread the frosting and sprinkle chopped pistachios on top.