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. Whisk this mixture with cake flour, baking powder, and baking soda in a medium bowl.

In a separate bowl, combine sour cream, whole milk, and vanilla extract. Whisk until smooth.

Cream the unsalted butter and granulated sugar in a stand mixer until light and fluffy. Add eggs one at a time, mixing well.

Gradually add dry mixture to the creamed butter mixture, alternating with the wet ingredients. Mix until just combined.

Pour the batter into the prepared pan and bake for 35-40 minutes until a toothpick comes out clean.

Allow the cake to cool in the pan for about 10 minutes, then transfer to a wire rack to cool completely.

For the frosting, blend mascarpone cheese, powdered sugar, vanilla extract, and lemon zest until smooth.

Frost the cooled cake with mascarpone frosting and sprinkle with chopped pistachios.

Slice and serve at room temperature, perfect with tea or sparkling lemonade.