Preheat your oven to 350°F (175°C) and prepare your cake pans by greasing them lightly and lining the bottoms with parchment paper.

In a medium bowl, combine the cake flour, baking powder, and baking soda. Whisk until evenly blended.

In a large mixing bowl, cream the unsalted butter and sugar together using an electric mixer until light and fluffy, about 3-5 minutes.

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

Mix in the sour cream, vanilla extract, and lemon zest into the batter until just combined.

Gradually add the dry ingredient mixture into the wet ingredients, alternating with the whole milk, starting and ending with the flour mixture.

Gently fold in the chopped pistachios using a rubber spatula.

Divide the batter evenly between the prepared cake pans and tap lightly on the counter to remove air bubbles.

Bake the cakes for about 25-30 minutes, or until a toothpick inserted into the center comes out clean.

Let the cakes cool in the pans for a few minutes before transferring to a cooling rack to cool completely.

In a clean bowl, combine the mascarpone with powdered sugar, heavy whipping cream, lemon zest, lemon juice, and kosher salt. Blend until smooth and creamy.

Once the cakes are cool, place one layer on a serving plate and spread a generous layer of frosting on top, then add the other layer.

Frost the top and sides of the cake with the remaining frosting and sprinkle with additional chopped pistachios for decoration.