01 - Preheat oven to 350°F. Grease an 8-inch round cake pan and line with parchment paper.
02 - Whisk together flour, chopped pistachios, baking powder, baking soda, and salt in a medium bowl.
03 - Rub lemon zest into granulated sugar in a large bowl to release essential oils. Add oil, eggs, and vanilla, whisking until smooth and well combined.
04 - Stir in the milk and lemon juice until fully incorporated into the wet mixture.
05 - Gradually fold the dry ingredients into the wet mixture until just incorporated, being careful not to overmix the batter.
06 - Transfer the batter to the prepared cake pan and smooth the top surface evenly.
07 - Bake for 30 to 35 minutes until a toothpick inserted into the center comes out clean.
08 - Allow the cake to cool in the pan for 10 minutes, then remove and transfer to a wire rack to cool completely before frosting.
09 - Beat cold heavy cream in a large bowl until soft peaks form.
10 - Add mascarpone cheese, powdered sugar, and vanilla to the whipped cream. Continue beating until smooth and spreadable, taking care not to overbeat.
11 - Spread the mascarpone frosting evenly over the surface of the completely cooled cake.
12 - Sprinkle chopped pistachios and fresh lemon zest over the frosted cake as desired.