Preheat your oven to 350°F (175°C) and prepare an 8x8-inch baking pan by greasing it with butter and lining the bottom with parchment paper.

In a food processor, finely grind the pistachios and lemon zest. Combine this with cake flour, baking powder, and baking soda in a mixing bowl.

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

Cream together the softened butter, granulated sugar, and a pinch of salt until light and fluffy.

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

Gradually mix in the dry ingredients and wet mixture alternately into the butter mixture, starting and finishing with dry.

Pour the batter into the prepared baking pan and bake for 35-40 minutes.

Let the cake cool in the pan for 10 minutes before transferring to a wire rack.

Mix mascarpone cheese, sifted powdered sugar, and lemon juice until just smooth.

Once the cake has cooled, spread mascarpone frosting on top and sprinkle with chopped pistachios.