• Preheat your oven to 350°F (or 180°).

  Spray an 8×8 metal square baking pan, line with parchment paper and set off to the side.

MAKE THE CAKE:

• In a mini food processor, add pistachios and the lemon zest. Secure the lid and pulse until finely ground.
• Add the finely ground lemon pistachios to a mixing bowl along with unbleached all-purpose flour, baking powder, baking soda and fine salt. Whisk to combine and set off to the side.
• In a liquid measuring cup, measure the whole milk and add in sour cream and vanilla bean paste (or vanilla extract). Stir or whisk to combine and set off to the side.
• In the bowl of your stand mixer, add in the sugar and butter. Mix on medium speed for about 3 minutes until light and creamy. Then on medium-low speed, add 1 egg in at a time, mixing well after each one. Scrape down the sides and bottom of the bowl as you go.
• With the mixer on low, alternate adding in the dry ingredients and the wet ingredients, beginning and ending with the dry, mixing until just incorporated.
• Spoon the cake batter into your prepared pan and spread evenly using a offset spatula. Bake on the middle rack of your preheated oven for 35 to 45 minutes or until a tester comes back clean with a few crumbs attached.
• Allow the cake to cool for 10 minutes in the pan before removing (using the parchment paper) and transferring to a wire rack to finish cooling.

MAKE THE FROSTING:

• In a medium to large mixing bowl, sift in the powdered sugar. To that, add the softened mascarpone cheese, lemon zest, sea salt, heavy cream and lemon extract. Using a hand mixer, mix on medium to medium-low speed until combined. Do not over mix, as it can separate/curdle.
• Once the cake has cooled, add the frosting on top of the cake and spread evenly with an offset spatula. Sprinkle with extra chopped pistachios before slicing and serving.