Preparation

Make the meringue layers

• Position racks in the upper and lower thirds of the oven and heat the oven to 250°F. Line 2 large baking sheets with parchment paper. Draw two 10×4-inch rectangles on each piece of paper, and then turn the paper over (you’ll still see the rectangles).
• Put the whole pistachios, confectioners’ sugar, and salt in a food processor and process until the nuts are finely chopped but not ground.
• Combine the egg whites and superfine sugar in the bowl of a stand mixer or in a large bowl. Set the bowl in a larger bowl filled with 2 inches of very warm tap water. Whisk by hand until the sugar is completely dissolved, 3 to 4 minutes. (To test, rub the whites between your fingertips; you should feel no grit.) Add the vanilla and the cream of tartar. Using a stand mixer fitted with the whisk attachment or a hand held electric mixer, beat on low speed initially and then, over the course of about 1 minute, increase to medium high and beat until glossy, stiff peaks form, about 5 minutes.

• Using a large silicone spatula, gently fold the pistachio mixture into the meringue. Transfer the meringue to a pastry bag fitted with a 3/4 inch plain tip. Pipe the outline of each rectangle and then pipe to fill in each rectangle, smoothing any gaps with the tip. (Alternatively, spoon the meringue onto the parchment rectangles and spread to fill.)
• Bake for 45 minutes. Swap positions of the baking sheets and bake until crisp and very lightly colored, about 15 minutes more.
• Turn off the oven and leave the meringues in the oven to dry completely, at least 1 hour. Remove from the oven and let cool completely on the baking sheets on racks. Carefully peel off the parchment. (If not using right away, store airtight at room temperature for up to 2 days.)

Make the filling

• In a medium bowl, whisk the sugar, zest, cornstarch, and salt until blended. Add the juice, yolks, and cream, and whisk until smooth.
• Melt 3 oz. (6 Tbs.) of the butter in a 2- to 3-quart saucepan over medium heat. Off the heat, whisk in the lemon juice mixture. Return the pan to medium heat and cook, whisking constantly, until the mixture thickens and comes to a full boil, about 6 minutes. Using a silicone spatula, press the mixture through a fine-mesh sieve into a bowl. Set the bowl in a larger bowl of ice water and cool, stirring occasionally, until the lemon curd is just warm (95°F to 100°F), about 20 minutes.
• Cut the remaining 10 oz. (20 Tbs.) butter into 1-inch chunks.
• Transfer the lemon curd to a stand mixer fitted with the paddle attachment (or transfer it to a large bowl and use a hand-held mixer). Beat the lemon curd on medium speed, adding one chunk of butter at a time. After all the butter has been added, increase the speed to medium high and beat until smooth, about 3 minutes. (The finished temperature should be 70°F to 76°F. If too cold or too warm, the filling will appear curdled. If necessary, whisk over hot or ice cold water briefly to adjust.)

Assemble the cake

• On a flat platter measuring at least 12 inches by 6 inches (or on a heavy piece of cardboard wrapped in foil), spread 1 Tbs. of the filling and top with a meringue layer. Using an offset spatula, spread with about 1 cup of the filling. Repeat with the remaining meringue layers and filling. Garnish with the chopped pistachios. Refrigerate for at least 24 hours to soften the meringue layers slightly before serving. Let sit at room temperature for 30 minutes before slicing with a serrated knife and serving.

Make Ahead Tips

The assembled, garnished cake can be refrigerated for up to 2 days. Let sit at room temperature for 30 minutes before slicing and serving.