Preheat your oven to 350°F (175°C). Butter an 8x8-inch baking pan and line it with parchment paper.

Pulse the pistachios and lemon zest in a food processor until finely ground.

In a medium bowl, whisk together the cake flour, baking powder, baking soda, and the ground pistachios and lemon zest until combined.

In a separate bowl, whisk together the sour cream, milk, and vanilla extract until smooth.

Beat the room temperature butter, sugar, and a pinch of salt in a stand mixer until light and fluffy.

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

Gradually alternate adding the dry flour mixture and wet mixture into the butter-egg batter, folding gently until just combined.

Transfer the batter to the prepared baking pan and bake for 35-40 minutes.

Allow the cake to cool in the pan for about 10 minutes before moving it to a wire rack.

Blend the mascarpone cheese, powdered sugar, and vanilla extract until smooth for the frosting.

Once the cake has cooled, spread the mascarpone frosting generously over the top and garnish with chopped pistachios.